Thanks LUNAT.
New update - after a chat with a friend of mine, who suggested getting a multimeter for the job. I am looking for a meter that is easy to use, any suggestions please ?
Pump stuck ? unscrew, rotate then screw back the 'hex knob'. Replace another fuse and this time, no...
Coming home from work to find the boiler has stopped working. Fuse F1 on the Printed Circuit Board (PCB) has blown - Inspect the PCB to find no other suspected faulty components (the local engineer had resoldered a dry pin around 18 months ago).
Replace the fuse; check the pressure gauge...