We only got our boiler replaced this year when it was 29 years old. And only because the replacement parts were no longer available. It was serviced annually, very clean inside (service engineers always commented- apparently it’s a good thing!) and we only spent about £35 pcm on gas. 12 years old for a boiler (if a good make!) is fine. So to ask for a reduction on that basis might be CF territory.
To be honest you can only insist on reductions for things you couldn't be expected to know from viewing and are a problem. I don't think the boiler is a big problem, the roof sounds like it is and you wouldn't have known, do it might be worth asking for a token amount.
Our boiler was a valiant and was nearly 40 years old! We only had it replaced because of a major refurb. They were made much more solidly in those days, and no computerised parts to malfunction. The manufacturer's would have you believe that a 10 year old boiler is old and needs replacing, but then they would, wouldn't they?