Mastermind is coming back and the BBC has asked me to chair it. It has great appeal for a political interviewer — and I’m not talking about the sadistic pleasure of shining a bright light into the eyes of the poor sod trembling on the other side of the microphone. Not that I would turn down the chance of doing that on Today occasionally. No, the real joy of it is there is no ambiguity. Either the answer is right or it is wrong. And if they don’t know or they won’t say, they can’t spend 10 minutes trying to kid you into thinking otherwise.

Political interviewers have modest enough aims. They try to establish what the politicians really mean, to test their arguments and to find out whether they have thought through the implications of their policies. It is not unknown for them to say different things to different people at different times.

Take Tony Blair on Iraq. Last summer, when he was trying to build a broad coalition, he said the war was all about ridding Iraq of weapons of mass destruction. When that began looking a bit dodgy the emphasis shifted. Saddam Hussein was a monster and we had to free the Iraqis from oppression.

Or take Iain Duncan Smith. He tries to persuade voters worried about better public services that they should vote Conservative because that is his main concern, too. He tries to persuade voters more worried about taxes going up that his is the party of low taxation.

Now there is another issue coming up — a real humdinger. It will stretch interviewers to the limits. It is the euro.

You might think it has been kicked into the long grass, and in a sense it has. In the next few weeks Gordon Brown will deliver the Treasury’s magisterial verdict on the famous five economic tests. We all know — or we all think we know — what has been decided. When Emperor Gordon stretches out his arm his thumb will point down. We, the citizens, may still get a chance to cast our own votes — but not in the immediate future. The job of the interviewer is to try to illuminate the role the political dimension will play when we do.

We have become so accustomed to talking about the euro in economic terms that we have tended to lose sight of the political arguments. That debate has stalled. The euro supporters, frustrated by the Treasury’s stranglehold, are anxious to get it going again. Mr Blair himself is rumoured to be on their side. All of which is fine — except that the closer you look at the political arguments the more you wonder whether either side has actually worked out what they mean.

At face value the government’s position is straightforward. It is in favour “in principle” of Britain joining. The only obstacle in the way of a referendum has been the economic tests. The political case, to use Blair’s word, is “overwhelming”. What could possibly be clearer?

Well, the government also says we should not join unless the economics are “clear and unambiguous”. So in other words, if the economic case falls even a little short of that test we should stay out. But hang on. They tell us the political case is “overwhelming”. That’s a big word. Could it be that they don’t know exactly what they mean by it?

We can guess what they might mean. If we don’t join the euro we will lose influence in the councils of Europe. We shall be made to leave the room when the important decisions are taken. Our ministers will have to stand outside in the corridors scuffing their shoes on the floor and waiting to be called back in to learn our fate. Dire indeed.

But that can’t be so. Denis MacShane, the Europe minister no less, has been telling us that the influence we have in Europe “has nothing to do with whether we pay our bills in pounds or euros”. Blair has been even more explicit. He said: “We absolutely have not and do not need to be a member (of the euro) to make our position strongly felt.”

Of course they might be just covering their own backs. No serious politician in power would admit to weakening his country by failing to take a given course. And anyway the evidence for that is pretty thin. Does anyone seriously believe Blair would have been able to persuade France and Germany to support war in Iraq if we had been in the euro? Or that we could have prevented them stitching up a deal to prevent serious reform of the hated CAP?

What Blair may have been acknowledging is what any experienced diplomat will tell you. A nation’s bargaining power depends on the strength of its case, the assets it brings to the table and the threats it is able to make — however subtle they may be. What many of our continental friends pray for every night is that Europe will grow into a sufficiently powerful entity to challenge the might of the United States. It seems unlikely that Blair had a mighty United States of Europe in mind when he said the political case for entering the euro was overwhelming.