OP: If the flue is "not to standard", it probably would not be "condemned". We "condemn" appliances that are "Immediately Dangerous", and ultimately will either disconnect or invoke a process whereby the gas emergency service will . (We have no right to disconnect against your instructions, the ES have a duty to do so) . If the boiler has a fault that is likely to present a risk to life or property, we will advise as such, attach a label, and turn off in a manner whereby you can take the resposibility of reinstating it. If the flue has a minor fault, we may just mention it or make a note on the paperwork.
Any pro here would strongly advise you to have the boiler serviced and checked before you use it. You do, of course, run the risk of it being found dangerous. But that is a lot better than the alternative.
So, really, the question is : would a defective boiler be a deal breaker? If so, you must insist that the boiler is checked before completion, regardless of who pays. I am not a lawyer (where is he, BTW
), but I believe that it would be an offence to sell a property knowing that there is a life threatening appliance in residence.
I personally think it would be innapropiate for you to insist on a new boiler, as you are aware that the existing one is a little past its use by date, but the vendor may be happier meeting you somewhere in the middle. But you must still have it checked before use - and that is the advice I give regardless of your specific situation. If the boiler was condemned, could you cope without heating, and is there alternative sources of hot water.?
All food for thought, but I get the feeling that you actually know the answers.
For what it is worth, I beleive a 552 would be at least 25 years old, but there are many (an older models) still chugging away happily