have a combi which gains pressure from 1 bar to 2-3 bar overnight
and have to release pressure

It will probably one of two things.

1. A leaking (or partially open) tap on your filling loop. These must be
kept disconnected, but almost never are. If disconnecting it shows the tap
to be leaking slowly, it must be replaced. (It might just not be turned
off).

2. A leak in your heat exchanger. This allows mains water from the DHW side
to cross into the primary circulation water in the combi DHW heat exchanger
(or DHW cylinder as appropriate). This is more serious and expensive, but
MUST be fixed immediately.

Christian.