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Utopia may be unattainable but that is never going to inhibit a part of humankind from driving in its direction. The way in which we talk of this perennial urge is important. If the goal is understated, if the utopian aspiration is modestly represented as practical and specific, why then, it seems reasonable and trustworthy.

The unhappy associations of starting from zero – clean slate, megalomania and hubris – are removed. So, the laudable ambition to, say, improve our cities is uninfected by such qualities. A plan may be touched by "vision" but not by "a vision" – the introduction of that unassuming indefinite article suggests that someone is downloading messages from the clouds. Unqualified "vision", on the other hand, signifies clear thinking, pragmatic intelligence, problem-solving nous. Unqualified "vision" is an unqualifiedly good thing. It is earthed. As earthed as the seductive conceit of linking Primrose Hill to the South Bank in London in the form of a continuous promenade. "Vision" in this instance belongs to Sir Terry Farrell acting in his capacity as a tardy ambassador from John Nash who's been pushing up daisies in the Isle of Wight these past 169 years.

Farrell's proposal, recently launched and debated at the Institute of Contemporary Arts in London, is to rehabilitate the spine of central London – which was as much Nash's creation as Regent's Park and Carlton House Terrace. And although those parentheses remain, what stood between them has all but disappeared – just as acknowledgment of Nash's supreme achievement disappeared for a more than a century after his death. Nonetheless, the course of his route through London is unchanged. It possesses a potential for coherence that is seldom found elsewhere in the city. But then no other architect so much enjoyed the patronage of the only British monarch since the Restoration to have built on the scale of a European autocrat. And therein, I'd have surmised only a few years ago, would have resided a ghostly hurdle. It is the atypical monumentalism of Nash's planning, its un-Englishness, which caused it not to be so much disliked as misunderstood and overlooked. We may heve paid lip service to the architect-planner's beau idéal of squares and boulevards and the public space they afford, but we were seemingly reluctant to use such space either recreationally or commercially.

The tentative provision of usable public space in London has created an undoubted demand

Farrell is far from the first architect to have championed the greening and sweetening of central London. But he is the first to have the breadth of ambition to appreciate that the Paddington-Euston-St Pancras-King's Cross axis of evil toxins is not lost forever. And beside that, Park Crescent to Waterloo Place is a cinch.

What truly distinguishes him from his precursors is his serendipitous timing. Mayor Ken may not be a European autocrat but he is a strategic dirigiste in a tradition that derives from the bosses of contemporary European cities (Juppé in Bordeaux) and from the great Victorian cities (Chamberlain, obviously). Second, travel has familiarised Londoners with the delightful Europe-wide practices of hanging out, lounging around and ambling aimlessly. The tentative provision of usable – as opposed to tokenistic – public space in London has created an undoubted demand, an appetite.