I agree with you, Simon, the Hexagon Housing building is rather striking, and it did replace a drab petrol station, but it is not quite as simple as that. The petrol station replaced this group of the cottages:

These cottages were perhaps three hundred years old and may not be "well designed or ... fit for purpose", but they are charming and historic nontheless. They were demolished in the 1960s (the film poster shows that
Stranglers of Bombay (1960) and
Kill Her Gently (1957) were being shown at the ABC).
While the cottages were there, and to a lesser degree when the petrol station was on the site, there was a partial view of the side of All Saints Church (a listed Grade II building). The Hexagon building has almost completely obscured that view. This is unfortunate, as the only visible side of the church is the uncompleted, and not very attractive, west end.
Furthermore, the buildings along Sydenham Road, between Hexagon Housing and the old chapel on the corner of Trewsbury Road, are all 18th century, although some may not look it, and nos. 122-124 are also listed Grade II. The bulk of the Hexagon building, in my view, rather overwhelms these buildings.
Paddy Pantsdown wrote:I would allow, nay encourage, anyone to demolish any building in principle including Buckingham Palace (or maybe especially Buckingham Palace) with one proviso:
** The replacement building must be of greater architectural merit **
This oft repeated phrase was perhaps first, certainly most famously, used by Sir Nikolaus Pevsner. In The Buildings of England: London he devoted 8 pages to Buckingham Palace, describing it as "supremely English" and pointing out its strengths as well as its weaknesses.
Paddy Pantsdown also wrote:We would not have hardly any of the great buildings of London if they had been fossilised in conservation areas.
Vast swathes of Westminster, Camden and Islington are designated conservation areas and even the square mile of the City of London has eighteen separate conservation areas. That has not prevented these boroughs from having some of the most exciting modern buildings in London.
A conservation area, as defined by the OED, is "an area deemed to be of special architectural, natural, or other interest, whose character and appearance are protected (usu. by law) from undesirable changes". This does not prohibit desirable changes.
Is it, perhaps, restoration that you feel so strongly about? The OED defines that as "the process of carrying out alterations and repairs with the idea of restoring a building to something like its original form; a general renovation."
It is worth remembering that very many of the best modern buildings, both here and abroad, were commissioned by public bodies of one sort or another. Anyway, all this talk of Bilbao, Buckingham Palace and Tate Modern is irrelevant to Sydenham, and I'm surprised that Admin hasn't stepped in to call us to order.