We are looking to buy a house and there are no building regs for the partial removal of the chimney breast.?

If money is tight, and you can't afford the full homebuyers, then phone several chartered surveyors and get a quote for them to inspect the chimney. Expect to pay a couple of hundred pounds, and shop around because costs can vary dramatically. For that you will get a report saying what, if anything, needs to be done, and you will get an estimate of the cost. You can use that as a bargaining tool.