I’ve asked a couple of installers/plumbers about this specifically and as to whether or not they should be serviced regularly.
Both said it’s not worth either servicing or insuring. Better to put aside the money you would spend for repairs if necessary.
We we’re in a flat for 12 years and spent £300 on repairs in that time for the already old when we moved in Worchester boiler. The boiler was never serviced.
We’re now in our house 2 years with a newish (think around 4/5 years old) potterton boiler. Have spent nothing on this yet.
Both plumbers we’re friends of friends who install/service boilers.
Take from this what you like, I plan to continue spending nothing on our boiler until it breaks down, but then I’m not a great believer in insurance in general.