Roger Mills wrote:
In an earlier contribution to this discussion,
Chris wrote:

Thanks for the replies ... Have since found out that the unit is a
Drayton Tempus 7 and on replacing the fuse again - the unit displays
the time etc. OK which you can programme but blows the fuse when the
hot water is turned on by "advancing" or by timing. ( No central
heating turned on)

Assuming it's a fully pumped system, I'd put my money on the pump. This is
the point in the system where water and electricity are at their closest -
and if a fault causes them to get *too* close, blown fuses (or tripped RCDs)
are likely to result.
Fixed - Leaking gland nut was dripping water onto the pump circuit and
blew the circuitry.

Thanks for all teh help

Chris