Combi boiler keeps needing resetting every couple of days!?

[Hottub]

Hello all, my Raven heat condensing combi boiler keeps needing resetting every couple of days, then works fine again?
If you run the hot tap with the heating on it works fine, however if you run the hot tap with the central heating off it puts the red light on and once reset it all works fine again for a couple of days? The heating/radiators all work fine it just seems to be when running the hot tap!
Does anybody know what might be causing this?

 

[ELZ4742]

diverter valve?

 

[Hottub]

That was quick! many thanks!!! do you know if this is expensive? and something I could fit myself?
Once again many thanks