Gosh - having gone to bed last night thinking my question answered - Canonbie Rd is in Forest Hill - this morning it seems more uncertain again! If there aren't any parish boundaries or anything that resolve it, maybe the answer really is that there is no definitive answer, as someone suggested earlier, so that it's largely a matter of usage? Perhaps it might also be the case that the areas overlap - I noticed this morning that the church on One Tree Hill, which seems like the epicentre of Honor Oak, actually calls itself St Augustine's Forest Hill! What's that about? Perhaps in the interests of diversity it would be good to try and hang onto the Honor Oak name as it's less well known (even people in Clapham and Brixton don't seem to have heard of it but do know roughly where Forest Hill is) and might be in danger of fading out of use given the greater prominence of Forest Hill.

Situation reminds me a bit of an article I read about Stockwell, where we lived before we moved here a few weeks ago. Apparently its boundaries have been shifting progressively northwards over the last 150 years - for anyone who knows the area, it started off very close to the centre of Brixton, as an area centred round Stockwell Green, but the southern parts of former Stockwell are increasingly described as Brixton or Clapham, and Stockwell is moving further and further north into the Oval and Vauxhall areas. So maybe if its not a borough, postcode area, parish etc., the main question is what most people call an area?