I like your optimism Haven't fully decided that yet. My proposed approach is to get the quotes first and see where we stand. Then negotiation I guess. If they say flat "No" to any negotiation then it really depends on the quotes. If its in region of 5K, I'm not bothered. At around 10K I'm getting bothered, 15K I'm back to renting I think.
Being an optimist, however, even if the repairs came in at 15K and the seller took a bath on that, they are still getting 15K over the asking price. Ive been going round and round in my head with scenarios/what ifs/what have yous, so Im trying to just be rational and do it in stages. Quotes are first step.
First thing is not to panic. I want you to be very calm and rational. You are too hung up on the asking price, it's irrelevant. The fact you paid 30K more would signify that it was a bidding war. It is a seller's market. You don't want to lose the house. You have the legals ready to go, the survey done and finance in place. This is a big purchase. You shouldn't lose it for 15K.
So please think about all of those things before you go on any high horse.
You are not screwed if you end up paying for this on top of the asking in a situation where it is a buyer's market/the sellor has another buyer. But you do have the sellor in the sense we're at contract stage with a good buyer who is ready to go. So sure, try and get all or some of the 15 K off the asking price (there is no problem changing the contracts, happens all the time, solicitor will just modify the figure with a pen).
Re the roof. All roofs need replacing eventually. You get a brand new roof that will last you a very long time. It might not need to be done immediately. Ask the surveyer how long does he think it will last.