oh and if you get guarantees for things like damp proofing get your solicitor to confirm the company still exists. I fell foul of this one a few years back on my 1st flat purchase. Sounds good being told by the vendor they still have 10 years left on the damp proofing guarantee but it wasn't until I was going through it to claim last year that I noticed the guarantee was supposed to be signed over to the new owners which it hadn't the last 2 times and the company had gone bust many years ago anyway.
When we had the work done again we discovered that the original company hadn't actually done the work they claimed to have done as there was no injection holes when the plaster and render were stripped off.
being installed by a gas safe installer is very important as it's your safety that is at risk and your insurer may not pay out. I have seen some very dodgy and illegal combi boiler installations over the last couple of years. If in doubt then budget to have it brought up to standard or completely reinstalled. Also if the building is listed in any way then make sure any new combi boiler installations (and other works) have had listed building consent applied for.
We have an installation in our shared building that had no listed consent, was installed through a wall we had just paid to have re-rendered and sealed and unsafely with a 12mm dia rubber condensate drain that was allowed to form a U bend outside which leads me to believe it was a DIY installation as no registered fitter would do that.
EDIT: Just read the bit about the original owner being deceased. Offer (in a nice way) to pay for an installation check now and see what they say. Would at least give you peace of mind. There should be some details of who installed it inside the boiler casing.