Fair play to MMC who have checked out the ECU, found no issues with it (as it worked fine on their "donor" vehicle that they do the ECU flash with) - and got it back to me very quickly, with a direct phone number to call if/when I needed further assistance.
I refitted it to my +4 as soon as DHL delivered it this morning and lo and behold, it works! The starter fired up the engine, much to my delight and relief, but also confusion - as we don't really know how or why the fault has been resolved. I was expecting to have to trace a dodgy connection in the starter circuits somewhere, under the direction of the MMC technical dept, over the phone. I called them anyway to thank them and tell them the good news - they suggested taking it to the factory service dept sometime for them to check over. We will see if the fault recurs, which obviously I hope it doesn't, especially given I have no intentions of meddling with the contents of the fuse/electrical box contents ever again - apart from checking/changing fuses.
On an even brighter note, I took it out for a quick half hour drive in the warm sunny weather and it drives rather better with the "enhancement flash" - Throttle response seems more progressive and less over-sensitive pulling away. I'm looking forward to seeing how it feels and drives on longer trips.
The MMC service department told me that although they will offer this enhancement flash service on GDI +4s, it will have to be done at the factory rather than by sending the ECU to them like I, and others, have done - the reason being that they will soon not have a donor car on which to carry out the work. The cost will also be going up - how much was not mentioned, but I paid £300. The fact that they kind of admitted that the "enhancement flash" is how it should have been set up in the first place, rather than using the Ford standard version, is a bit irksome to me. But then I suppose that running a Morgan was never going to be cheap...
