I would first check if you have separate mains feeds to boiler and wiring
box. If separate I would not suspect boiler or pump initially. The
intermittency may just be due to the actual current being drawn. Don't
have the tables to hand but depending on the actual current the fuse may
take some time to blow. If you are confident (and competent) I would
suggest inserting a test meter on ac Amps in series with each load (zone
valve(s), pump, boiler etc). Obligatory health warning - switch-off while
connecting. What I might do in this situation is to fit a 13A plug (3A fuse)
to the feed and plug into a 30mA RCD. Healthy readings should be nowhere
near 3A. OK, won't work if it is genuinely an intermittent, but I would
suspect not.

*********UPDATE*********

Had a bit of time today so this is what I did. Got a card of fuse wire
- cheaper than 50p a go fuses, even if a bit fiddly!! I turned off the
power and replaced the fuse.
I disconnected the red wire feed to the pump in the wiring box.
I set the programmer to off for both CH and HW and turned the power
back on. All ok so far.
I then switched the programmer to call for HW, still ok.
I then switched the programmer to also call for CH, still ok.

Eureka I'm thinking...it must be the pump - Grungfos 15-50. Based on a
previous post I tried to get hold of a replacement capacitor but no
luck. Decided I'll replace the pump, on the basis that even if it
isn't the pump this time at least I'll have a replacement next time.
I turned off the gate valves eitherside of the pump and tried to
remove the pump. Unfortunately I didn't have large enough spanners to
remove one side of the pump so reconnected to pipework again. What I
didn't realise was that whilst I had the gate values on either side of
the pump disconnected, the boiler couldn't come on. Presumably this
means I have a fully pumped system?

Anyway I decided to leave the pump disconnected this evening and try
again tomorrow. Bear in mind that at this time I'm still thinking its
the pump. The boiler was on and happily heating water and
approximately 40mins to an hour later the fuse went pop again.

When I was in the plumbers merchants he said if the pump was gone it
would smell of burning - it doesn't.

So now where do I stand? Have I confirmed it ISN'T the pump?
Does the fact that it took 40mins to an hour to blow indicate the
blowout happened when the motorised valve was moving to a different
position once the HW was heated?

Back to you boys ot there.

Charlie

And it was ok foabout 40minutes. The fuse then