The gifts of the magi, readers of the famous O. Henry story about Christmas in a long-ago recession may recall, were well meant but useless. Jim gave Della a set of combs for the long hair that, on Christmas Eve, she cut off and sold to a wigmaker to pay for his present - a chain for the watch that he had pawned to pay for the combs. This fond and foolish couple would have been safer buying each other underwear.

Well, probably not. The only time my wife bought me underpants I was undelighted to discover that they came with no fly: a curiously emasculating attempt, I felt, to improve my bedroom confidence. And as for men, they are apparently so rubbish at buying their women bras, knickers and negligees, that John Lewis - to whom all right-thinking women, including Gillian Anderson, Ulrika Jonsson, Patsy Kensit and Fiona Bruce, trust their most intimate measurements - has set up a “lingerie academy” to teach us a thing or two.

It runs next week (December 17 to 19, noon till late) at its Oxford Street store in London and is, obviously, a bit of a gimmick. To tempt us into this inner sanctum of womanliness, the lingerie department will be “transformed”, its PRs exaggerate, into a “gentleman's oasis” consisting of black leather sofas from the furniture department, plasma TVs, Wii and free beer. It's very Playboy Mansion, although to be honest they had me at “lingerie”.

But the idea behind it is sound enough. No one admires underwear more than your average bloke - I have even read a book on the subject, the Playboy Book of Lingerie - and no one knows less about it. What is the difference between full cup and padded, balconette and plunge, underwired and multiway? These are areas of the purest ignorance to me, the unknown unknowns in Rumsfeld-speak. But when it comes to the known unknowns - such as “will this bra fit my girl's breasts?” - many of us are just too scared to ask.

Maria Walker, the shop's chief furniture fitter, says that we shouldn't be. Her assistants are always happy to share their expertise. On academy days they can be identified by their pink T-shirts bearing the legend “Find the Perfect Present”. The confused male customer is just as easy to spot. “They come in and they are like deer in headlights,” Maria says. “And you always spot the ones who have never done it before. They look a bit lost.”

I think we feel a bit pervy, I suggest. “I know a lot of men worry about that, but once you are in a lot of that tension and worry disappears because we are here to have a bit of fun too. It is not too serious.”

Unless you buy the wrong thing.

“The mistake men most commonly make is buying for themselves and not for their partners,” Maria says. “They buy something they would like to see their lady in and she wouldn't be seen dead in, usually something racy in red or black and lace.

“I saw a programme recently called Britain's Worst Husband and these men were sent into a high street shop to buy lingerie and they all came back with red and black lace. Every one. We have some beautiful things in red and black, but you have to make sure of the quality and you have got to look at your partner's colouring. Someone with dark hair and olive skin would look wonderful in red but if you're blonde...”

My assignment is to pick a Christmas present for my wife that does not make her think I think that she is (a) a frump, (b) a tart or (c) two sizes bigger than she is. I come armed with her measurements - which I will divulge to Maria but not to you - and her prejudice against frills and fiddly bits that show through her sweaters. But beyond that, as I view the ocean of lace and cotton that is the John Lewis knickers department, I realise I am all at C-cup.

How much is that, I ask, looking at a teeny-weenie padded thing that calls itself Elle Macpherson. “32,” Maria says. I say I meant the price not the size. She says she meant the price: £32. It seems a lot to me but we are still at the cheaper end. We plunge on and reach the Legaby range, where the stitching is by hand and there is no change from £55.

How much do they go up to, I wonder. “K-cup,” Maria says, misunderstanding again. “Fantasy,” she says. Absolutely, I agree, but she means Fantasie, the brand, there to serve the extravagantly endowed older woman (the voluptuous younger vixen will probably choose Freya). I ask if Maria has ever been defeated by a lady's upper hourglass. She thinks not, although sometimes a bit of extra stitching may be necessary. I pick up a Fantasie. Some of these flimsies, I remark, have the tensile strength of the Forth Bridge.

I suppose that she sees more fantasy busts these days what with plastic surgery. “We have Harley Street round the corner so we get lots of people coming in before and after surgery. Some of the clinics send them straight here.”

And what to do if a bloke turns up with no idea of his other half's “vital statistics” before or after? Maria assures me that it is acceptable to compare her physique to the shop assistant's. She is unlikely to sue for sexual harassment. But the safest thing is to stay away from bras and briefs and consider a camisole or chemise instead. And if you even suspect that she thinks her bottom is too big, don't buy her a thong. Every bum looks big in a thong.

She shows me something racy called Deadly Nightshade. It lies at the luxury end. I'm thinking “mistress”, I say; surely no woman would buy herself anything like that. Not so, she says, leading me to a black-and-red own-brand John Lewis bustier. “A lady might want to wear that on a first date,” she says. Does many a happy marriage begin in her department then? “It might be a good place to start.” But where might it end? In the long fleecy nighties that John Lewis still sells? Beneath their deep folds decorated with leaf and berry stencilling, everything would be left to the imagination. And before then, after the first flush but before all passion is spent? I quite like the Cyberjammies: jolly pastel PJs, more Doris Day than Jordan.

Some men really do apparently buy one set of undies for their wife and another for their mistress. “We had a guy last year who bought exactly the same set for both but in different sizes. He was quite open about it. One for his wife and one for his girlfriend.”

Do blokes ever buy for themselves? “To wear, you mean? Yes, we do get a lot of transsexuals and transvestites and we do fit them. We have not got a problem as long as it is a booked appointment. It is a service we do. I think everyone is up for a bit of a laugh now.”

Making it clear that I am not, I wonder if it is time to make my excuses. I leave with what I hope is a very fine gift: a green silk padded John Lewis bra and accompanying knickers, kindly donated not so much as a bribe as a test case. I am told they will suit my wife's pale colouring and dark hair, that they are bumpless. I find them fetchingly Christmassy. I also leave knowing more than I did: that camiknickers and teddies are passé, that there is a Brazilian knicker, that the trend is towards brighter colours, that Playtex has moved on from the Cross Your Heart to the Wonderbra and that you should never buy a blonde a red one. And those who predicted that the thong would remain the same did not count on the vagaries of fashion.

At home I give Lucy her early Christmas gift and stand back for the explosion. Have Maria and I avoided the pratfalls? “It's green,” Lucy says a little shocked, a little awed. But it is a perfectly sexy fit. “Lovely, darling,” she says, and I think she means it.