boiler pressure rising worcester

Hi, I have a Worcester Bosch combi boiler. I'll update the thread with model when I get home. It's like the 24i except it has 2 dials for CH temp and HW temp. I noticed the pressure gauge was in the red. I went to check one of the rads which usually gets air. No air. I drained one of the rads...

Exchanged the pressure relief value today but now when we put the heating on the pressure won't stop rising and boiler is shut down by safety switch. Pressure stays constant when heating off so have hot water but as soon as heating goes on it just shuts down. Any ideas please.