At my old house I used a local heating oil company, SS Motors. As part
of their customer deal, they offered annual boiler servicing for a
token charge of about £35 (this was 2003).

On the only occasion in 4 years I had a breakdown, I paid a £12
callout charge - and the engineer had a second hand spare from an old
boiler he had removed - £5.

The company mostly serve Cambridgeshire.