Reducing an offer following survey & roof replacement requirement



New Member
Hi there
I'm looking for some advice on how to proceed with my buy on a London apartment. My offer was finalised as £15K over the original asking price. It has now become clear that there are major roof deficiencies, missing slates, rotten timbers, end-of-life parapet guttering, underfelting - the works, it needs a complete replacement and a Section 20 will be issued in due-course.
I would still like to proceed with the purchase, but from the figures i've seen the cost is likely to exceed £20K per resident.
Should I reduce my offer - and if so, by how much? Should I leave it until close to exchange so as to give my vendors less opportunity (regarding their chain) to look for another buyer?
Would be so grateful for some advice here!