Sime dgt HE 30 combi is leaking from the top and coming out bottom left corner, losing pressure slowly

Check valve is letting it bleed back.

Does it look like soot ?

I suggest you just buy a new one. I had the almost to exact same problem that you have. its going to be just as expensive to repair it to just getting a new one.
-Shianne :)

3 psi is a little over the average, 2.5 is the optimum setting. If it empties itself, it means that the safety valve is working,which prevents any kind of damage to pipes etc...Make sure the valve is open and close at the right time and top up the system slowly rather than in a hurry. If the system requires regular top up then there must be a small leak somewhere, so it is best if you can fix that too. Remember that there are some solutions that are added to the water circulation and if you are constantly topping up with water then you are losing these solutions that prevents rust build up etc...Hope this helps.

All the combis I've come across merely fail to fire if the pressure is not up. not only do they not fire, nothing else works including the pump and in any event the pump should not overheat just on low pressure.
water loss - try turning off the valves on the flow & return pipes [3/4 inch ones] 1 valve per pipe . This will isolate the rest of the system - rads etc Note pressure over a time period. In this mode the hot water circuit will still work but do NOT use the heating circuit
1/2 bar over 2 days is not serious but it should be found
Once you have either mended the leak in the rest of the system or found there is not one, insist on a replacement boiler
In any event having looked at the reasons given for the fuse blowing, I would definitely get a replacement.
These problems should not happen on a new boiler
I list Worcester.-Ariston-GlowWorm-Ideal- and many others who would confirm this

what kind of heater combi? if combi you may be pressurising boiler to high ie over 1 bar boiler may be blowing off through release valve check discharge pipe

check the tops of all your radiator valves for bypass if found, easy to repair best of luck

if your boiler is a combi re-pressurise locate filling loop and refere to manual

replace pressure release valve