Major works, £50k for damp-proofing one flat...

D_M_E

As Tom99 indicates in his post - if it was built in 1890 it may well not have originally had a DPC installed when constructed and therefore na DPC would be classed as an improvement for which you may not have to pay.

You need to carefully read and check your lease and see what it says about it - particularly if the building either does not currently have a DPC or if it's been converted into flats and a DPC was not installed at time of conversion.

Don't see why you should have to contribute towards refurbishment of the damp premises - would have thought that that would be the responsibilty of the leaseholder of that particular flat.

Hoploz

Seems to me it isnt the damp work that is the main costs it's the refurbishment. The owner of the flat should be contributing to this, however I would suggest that if the problem was reported 4 years ago then they could argue it has got worse through negligence ie it wasn't repaired quickly enough and there has been further damage not their fault.

Ultimately the question to ask yourself is, would you have bought your flat if it had been £6500 more than you paid, and if it's worth the extra now a few months on, then you haven't really lost out.