I believe it's an "unscrew the silver coloured ring" job. I hate having to do that, the access is so tight for adult hands. Putting it back together is miserable, too.
Chris - most likely is the same one. Let me see how this goes before I take you up on your kind offer.
To release the connector there is a small dimple button on the top. When depressed this allows the retaining cam to rotate forward. The connector will then lift off. Do not force it. The ECU connectors are keyed so that they cannot be wrongly connected. When refitting the cam mechanism pulls the connector onto the pins, again do not force.
Well...going to fess up for idiocy... @Chris really kindly dropped around last night with a spare relay...this did nothing to change the symptoms. I was showing him what the symptoms were and he said “It sounds like a flat battery, R”. Swapped batteries with my road R just now...all cured! Started just fine! Apologies for idiocy people but thanks for all the suggestions!
To be honest I'm a little disappointed with everyone on here. My first thought was battery but I thought I'd let you lot work it out between you.