The poet and artist William Blake was forever having visions. At Peckham Rye he saw a tree full of angels, and on South Molton Street – now one of London’s most chichi shopping thoroughfares – he saw the devil. “He was walking up the staircase here and – it’s a bit like a blues song – he met Satan,” said Niall McDevitt.

Pointing at the door of number 17, now an employment agency, McDevitt began to whirl his arms and declaim in an otherworldly voice: “It is the gothic fiend of our legends – the true devil – all else are apocryphal.”

A passer-by laden with glossy shopping bags paused, mystified. “Are you with us?” asked McDevitt. “This was William Blake’s house. England’s greatest poet? ‘Tyger! Tyger!’?” The woman looked blank. “It’s a bit of a happening, anyway,” he said drily as she walked off.

The happening in question was one of McDevitt’s regular guided walks through central London in honour of one of the city’s greatest sons. This year marks the 250th anniversary of Blake’s birth, in Soho, and no literary figure – bar Pepys and Dickens – is more closely associated with the capital than this awkward genius of a man.

Blake lived his life in London except for three years spent on the south coast, near Bognor. He was a London tradesman – he made his living as an engraver – and he knew “each charter’d street” of the place. Superficially, London has changed greatly since Blake’s day. Only one of the five houses he lived in in central London – 17 South Molton Street – has survived the demolition ball.

Yet the spirit of the time, and of the man himself, hovers still in the alleys and the air. To conjure them you need to follow Blake and trust to vision and spirit rather than “corporeal” reality. As Blake said himself, and McDevitt demonstrates on his walks, “He who does not imagine in... stronger and better light than his perishing mortal eye can see does not imagine at all.”

McDevitt ( “musician, poet, literary walker – but essentially a poet”) has more than a touch of the Blakean madman about him. Riffing on the fact that the rooms Blake and his wife, Catherine, occupied in South Molton Street now belong to the Reed employment agency, he spouted these lines from the Introduction to Songs of Innocence: “And I pluck’d a hollow reed, /And I made a rural pen...”

And so we continued, our group of 10 (including five unusually rapt A-level students from Burnham-on-Crouch), stumbling in McDevitt’s wake through the workaday West End streets as the man flung poetry and prose over his shoulder.

Blake was born at 28 Broad Street, now Broadwick Street, on November 28, 1757. The house, which stood at the northeast corner of Broad and Marshall streets, was demolished in the 1960s, and in its place is a ceramics shop, which is part of a brick tower-block complex. Here the toddler Blake had his earliest vision, of God outside the window: “The vision was sublime and terrifying and set the young child screaming,” said McDevitt.

Here, also, much later in his life when the house was his brother’s underwear shop, Blake put on the only solo exhibition of his paintings. None of the work sold, and the only press review dismissed him as an “unfortunate lunatic”.

Half-running to keep up with McDevitt, our visionary troupe skipped round the corner into Poland Street. At number 28, Blake wrote and engraved the deceptively simple cycle of poems known as Songs of Innocence. “There should at least be a plaque for that,” said McDevitt. But the bland facade – it’s now the Eclipse Hairdressing salon – gives no clue as to the beautiful things that were created here between 1785 and 1790.

Trotting across Leicester Square (Blake lived briefly on the edge of what was then Leicester Fields), we paused to mock the busts of Joshua Reynolds and Isaac Newton – two sirs whom Blake loved to hate – while Hogarth’s likeness prompted poetry. The artist who painted A Harlot’s Progress, and whose earthy depictions of London complemented Blake’s visionary ones, was treated to the lines from Blake’s poem London, about how “the youthful harlot’s curse / Blasts the newborn infant’s tear / And blights with plagues the marriage hearse”.

In the National Portrait Gallery, McDevitt astonished the Sunday-afternoon potterers by recounting with gusto Blake’s meeting with the Archangel Gabriel, who took off through the roof of the poet’s study and stood in the sun, beckoning. “Perhaps it accounts for the rapt expression in Blake’s eyes,” he said of the painting by Thomas Phillips, completed 200 years ago.

We completed the walk in a dingy alleyway, round the back of the Savoy hotel, called Savoy Buildings. In Blake’s day this was Fountain Court, where he lived the final seven years of his life, and died in 1827. In a fusion of the actual and the spiritual that Blake would have appreciated, two Polish chambermaids spilt from the Savoy staff entrance and sparked up cigarettes as McDevitt was describing Blake’s happy death and how his friend George Richmond had closed the poet’s eyes “to keep the vision in”.

The William Blake Walk, led by Niall McDevitt, starts by the jeweller’s shop at the corner of Oxford and South Molton streets at 3pm on occasional Sundays: for details of the next walk call 07722 163823 or e-mail thewilliamblakewalk@yahoo.co.uk ; £6 per person.

The National Portrait Gallery is open daily, 10am-6pm (Thu and Fri, 9pm); the Blake portrait is in room 18; his life mask, not currently on display, can be viewed interactively on one of the free screens

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