Had a chat with the agent in Kent yesterday afternoon and it's definitely looking more apparent our vendors are nowhere near ready to proceed. The sticking point is they don't have a building regulation certificate for the extension and although it's possible to apply for a regularisation it takes about 8/9 weeks. Even if they did decide to apply for one that's too late for us. The only other option would be if they accepted a drastically (I spoke to a lady at the local district building controls and that was her word) reduced offer to compensate for the risk we would be taking. The agent, who incidentally has not mentioned it before, was trying to assure me an indemnity insurance would put things right. Not true but I'm sure lots of people at this late stage will grab at any straw rather than lose the house they want. All an indemnity does is insure against the council insisting the extension's demolished, but as I've been assured that's not actually a danger - apparently the council only have two years to enforce this, again confirmed by the lady I spoke to - said insurance isn't worth the paper it's printed on.
In reality no one can be forced to seek regularisation, but once you do you must comply with whatever repairs/rebuild deemed necessary before the certificate is issued. Lots of people never bother and to be honest provided the structure is sound it's not a problem while you live there, but and it's a big BUT when/if you need to sell no lender will finance without it and of course there's no guarantee you can find a cash buyer willing to take the risk. We might, at the right price, but my guess is our vendors have known about this problem all along and have hung on until this late stage banking on us being naïve, desperate or both. Looks like we'll be in Kent next week looking for a rental