Why does my Trianco Eurostar oil boiller not run the hot water system?

I have a 15 year old Trianco Eurostar oil burner with a built-in Horstmann timer and additional hot water tank temperature sensor. While the central heating works fine, there is no separate hot water provided. It just doesn't run even though the HW indicator shows on at the correct programmed times. Is this problem likely to be internal in the programmer and I will need to replace it, or should I try a new tank temperature sensor first. Unfortunately the built in programmer does not appear to be available these days but that would be a cosmetic issue with the newer, larger ones. Is there anything I can try (possibly a fuse somewhere?) that may be a solution.