Finally got a qualified plumber out to look at this and changing the inlet control valve (https://www.unventedcomponentseurope.com/oso-multibloc-inlet-control-valve-22mm-355045.html) fixed the issue :-) I live in a hard water area and it was last changed five years ago.
Thanks regarding the note about Immersion Heater, it's definately off.
It's an OSO Indirect 250. I did what you said with the commissioning valve and water flowed out. I then closed the cold water feed and opened the Pressure Relief Valve as per the procedure and hot water flowed out from it...
Last week I noticed that the tundish was dripping and followed the normal procedure:
Cold or tepid water discharge from tundish
1. Turn off the electrical supply to the immersion heaters.
2. Turn off cold water supply valve.
3. Open a hot tap.
4. Drain water from commissioning valve (12) until...