Fan in WBosch condensing boiler suddenly stopped (12years old)

[Muddle2000]

This has never happened before It is serviced every year Lately is has been connected to a prepayment meter and gas has cut out maybe ( 4 or 5) times as it has not been topped up by tenant Could this starting and stopping caused the fan to malfunction

Possibly. Fans are moving parts and moving parts (valves, fans are normally the first things to fail on appliances). Most things that are on permanently have a greater risk of failure when switched on and off.

However, you would never be able to prove it. And it could just be caused by normal wear and tear on the fan. WB boilers are good quality, but things on them still fail - and to me 12 years is not a short time for a fan to fail. I doubt whether the service every year includes an inspection or replacement of the fan (beyond whether it is working or not), so it's hardly relevant.

Probably one of those times where you have to accept being a landlord costs money in wear and tear.