on 17/07/2019, John Rumm supposed :
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.

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.