trickydicky wrote:
The domestic hot water system works fine but when the central heating
system comes into play the pressure in the system gradually builds up
until it reaches 3 bar which activates the pressure release valve.

Inadequate air charge in the expansion vessel? The water can't be
compressed & can't absorb it's expansion when the system heats up.
Pressure rises to 3 bar, excess water is discharged. System cools down,
water contracts, pressure drops, boiler lock-out when next required.

See
http://www.makewrite.demon.co.uk/SealedCH.html