Change the leaking valve is the best course of action for you
Thank you - there was no movement at all in the pipes, so it would have meant dismantling other pipes. I therefore decided to try the approach in the video, as if it didn't work then I would need a plumber to dismantle it anyway.
The system is drained. I followed the video to remove the spindle/core. What is not the same, in case anyone else is considering this, is thr metal plate isn't attached to the spindle core - the plate is holding a unit in place. There is also a very fine black seal inside the two port valve rim between the spindle and the brass valve. I spotted it while taking the new one apart to see if it was similar to the video (it was broadly - so as he warned, also be careful to keep hold of the centre when removing the spindle from the body, or it could fall apart/spring out).
I removed the old centre, including the old seal, wiped it to clean out any potential dirt, put in the new seal and new spindle, replaced the cover plate. Water replaced in the system and it is not leaking.
However, the boiler is still not firing (after its initial routine when the power is switched back on). The boiler is not showing any error messages. MPE222 actuator changed in case that had broken due to water damage, but still no hot water nor heating.
It looks like I will have to call out a plumber after all, as I have no leak (which would appear the obvious issue, given the hot water was fine until this), but no working system .