Thanks for your reply.
The first rule about buying a property is that you take it in its existing condition and it is up to a Buyer to satisfy themselves over the structure/services/boiler.
However, it appears to me (and it will be up to you to prove) that the Vendors may be guilty of Misrepresentation, which is where a knowing untruth has been told, and which you relied upon in proceeding with your purchase.
It is up to a Buyer to therefore show that in effect the Vendor has provided an answer they know not to be true, on the Property Information Form, and as a consequence, you have sustained a loss (ie a new oil boiler). If you can prove this, then the Court would look to the Vendor reimbursing you for a new boiler, or at least an amount towards a new boiler.
Court proceedings are of course very expensive, if you were to instruct a Soliciotr, but this matter would be heard in the small claims Court and which court can be used by private individuals, without the use of a Solicitor. (You would need the Vendors current address to enable proceedings to be commenced).
Hopefully, therefore, a letter from your Solicitor threatening such proceedings will make the Vendor see sense and lead to them making you an offer. If not, the court route is the only way forward.
I hope this assists and sets out the general legal position.