I used to advocate my team repairing older boilers if the customer wanted it. We even had a guy who had been a BG specialist on Saunier Duvals that would fix any Saunier.
But regrettably, the great British public have changed my view. So many times we would go the extra mile and fix a fault on a miserable old boiler (oh, thank you, no one else would touch it blah blah).
8 months later, I would receive a call. 'Mrs Jones here, you fixed my boiler last year. Well its gone wrong again. I paid good money for that repair.' Mrs Jones assumes that your company have a picture of her on the wall and have only 4 other customers, you will remember her house and her boiler.
So you go round having replaced the divertor 8 months ago and the PCB or fan has gone. They want it fixed for nothing. If not they will tell everyone in the Bridge Club that you shafted them.
So, I'm afraid, if it is a combi and its old and looking crusty we tend to say 'get a new one'. Not because we want to fit it, but because someone owning a seriously crusty boiler and not wanting to change it is exactly the sort of person who will try and get free repairs forever after the first visit.
Frankly, they make it bad for the rest of the general public but we have to protect our company and staff from sharp customers.
I know Agile likes to think he can repair anything, well so do we. The difference is, we are careful about which individuals we offer this to. If the boiler hasn't had a service for 5 yrs, this tells you all you need to know.