Grimly Curmudgeon wrote:

If the pressure relief valve is the spring-onna-wotsit-wivva-red-cap
type, try quickly rotating the cap so it reseats. Often a bit of system
**** will clag the seat.
If that doesn't work just replace it, they're cheap enough. Dripping
from these is quite a common fault.

If the relief valve is leaking, the system pressure should gradually drop.
Once a volume of water equal to that of the expansion vessel has left the
system, which at a steady drop should be a matter of hours or days, the
pressure should have gone to zero.

That the system is nevertheless still at pressure (the OP said 2 bar)
would, I suggest, seem to indicate that there is a leak in the manually
operated (and normally fully shut) mains water top-up valve.

Come to think of it, isn't 2 bar a bit on the high side? Could it be
that the relief valve is actually doing its job perfectly, and that the
real problem lies *only* with the top-up valve?