Boiler Pressure Keeps dropping advice needed

So you have a leak somewhere. Iso valves don't always iso. Anything can leak. Sounds like you need a good plumber rather than gas engineer. A pressure gauge on the other side of the iso valves would help narrow things down. You can also put fluorescein in the water & use uv to help see where it's leaking. I'm not a plumber, there may be better suggestions from others.

NT

Be careful with fluorescein: a tiny speck can go a very long way, especially in the kitchen. DAMHIKT!

Yup, I have exactly the same issue with my boiler. Having checked all the possible pipework leaks, I realised it was in the boiler itself.

Turn off the boiler. Top up the pressure to ~1 bar if required. Isolate the CH flow and return. Now remove and empty the condensate trap[1], and replace. Leave for an hour or two and see if pressure has dropped. Check the condensate trap again. If there is a leak in the main HX, then there will be water in there (you may even see an occasional drip from the trap mounting point.

on 17/07/2019, John Rumm supposed :

Isolator valves don't usually completely isolate, they will still pass some water.

Last resort to trace the leak, is to drain the water and pump the system up with air pressure. Air will escape much more easily that water, so more volume and you will hear an hiss.

Gate valves yes, ball valves are much more reliable. I think I've got rid of any gate valves in this place...

It does not really matter - if you empty the trap and the boiler is not running, it should stay empty.

Thanks, will try trap test this weekend

If you do the test and the trap does not refill, remember to refill it manually, since its not good to run the boiler with an empty trap since there is a slight risk it could allow combustion products back into the room via the empty trap.

Having same bother with same boiler and the condensate trap has been clogged a couple of times with some crap that looks like sand.

Could the sandy crap be forming in the system somewhere,

Or could somehow the crap be being drawn into the condensate trap from the drain, is that even possible?

It happens that R D S formulated :

Likely airbourne sand from the Sahara. It just dirt drawn into the combustion chamber, washed down and out by the condensate. Thats why the trap is supposed to be checked and cleaned out as part of the service.

Finally got round to fixing mine:

The pressure loss I was flacking about in a self installed underfloor heating system (professionally installed boiler and manifolds) turned out to be two leaks at the manifolds. One was a cracked air release valve, probably due to rough handling the other a poorly fitted 22mm olive. Not apparent until the system was allowed to cool when the drips were obvious. Leak rate around 2 egg cups per day.