Fraud property purchase? – Defects not known to the seller

Not a lawyer, but expressing my opinion as a lay person.

You ask "Is it legal or reasonable for a person to buy a property in the full knowledge that they are buying it with known defects?"

I would hazard a guess that if someone wants to buy a lemon, then there is nothing to stop them. It can't be "illegal". However, it would be illegal to sell something with a known defect and not declare it.

Really the onus was on you to get your own survey done so you knew what you were selling. Whether he can now make a claim against you now having accepted the defects is another matter and only a lawyer can answer that. But it would appear that you are 50% liable in any case because of the joint freehold, irrespective of whether the buyer knew or not.

Presumably the work needed doing and if you had known about it prior to the sale you would have had to bear the full cost. Unless I have missed something you have benefited by half the cost being paid by the purchaser, so what are you complaining about?

I would look at the shared freehold agreement and see what it says. I hope that you had this drawn up by a solicitor.
It's perfectly fine for someone to sell a property with defects. This happens all the time, for example where a builder buys a tired old building and refurbishes it to sell on. They never know what they are going to uncover, and it is a case of 'buyer beware'. In your case, the buyer had a survey done so was either aware of the problem or should have been.
However, your leasehold / shared freehold agreement will hopefully say who is responsible for what.

It sounds as though your purchaser is being awkward because he hopes to buy the other flat. There is no issue with him deciding to purchase the flat with roof issues although it is a little odd that he would do so when he could have insisted you do the work first. Perhaps he didn’t wish to delay the sale. As has been pointed out, that has been in your favour. As far as the later behaviour in terms of having the work done without allowing you the opportunity to get your own quotes, that will come down to the wording of your lease and how reasonable it was given the urgency of the work. The fact that he was in occupation and therefore potentially endangered will be in his favour. Whilst I understand your desire to ensure the work had been properly carried out before paying, if the joint freeholder had already paid the contractor in full he’s not going to be happy to wait. Basically, you’ve now been satisfied that the job has passed building regs, so unless you feel the cost was substantially more than it should have been (and frankly we would need to talking about more than double to make it worth your while to quibble), then you really have no reason not to hurry up and pay him and get on with your life and the sale of your other flat. The joint freeholder will be happy to make it harder for you to find another buyer if he can. He’s already told your agent you owe him money, next he’ll tell them you have a neighbour dispute with him which the agent is bound by law to disclose to any potential buyers. They won’t want to live in close confines to a difficult person.

So, pay the man. Do so with a smile and thank him for organising the work so efficiently. Apologise for your delay in getting him the money and keep quiet about your true feelings of irritation. Don’t give him ammunition to be difficult & just hope he doesn’t play loud music during viewings.