I read this with optimism and was hoping I would be able to send you a bottle of your favourite tipple. 
Sadly, it didn’t work.
Thanks very much for your research and efforts in helping.
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The only other thing I can think of is to use FORSCAN software and an ELM327 usb obd cable.
This is how I programmed extra keys in to my ECU.
You get a 1 month advanced licence trial when you load the FORSCAN software so the only cost would be the cable. ( Mine was less than £20 )
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The only snag at present is my OBD 2 reader wouldn’t connect.
The chaps quite sofisticated one wouldn’t either. 
I think therefore I am currently in the hands of Mazda.
In fairness, he was extremely helpful and gave me some quite reasonable prices.
He was going to do a full diagnostic check first before anything anyway.
Time will tell and fingers crossed they can sort it.
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This thread seems to indicate that the obd port is only live when the “room fuse” is good and there is 12v to it.
His issue seemed to be the main 120a fuse.
I would make sure the room fuse has 12v to it and take a look at the main 120a fuse
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I never mess with wiring / electrical as never feel competent enough and when it goes wrong 
ball ache hopefully you find solutions Richy TDA
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Well, well well!
Been at the Mazda dealership in Bath for a week.
They’ve checked rather a lot of stuff and connections etc.
Likewise, been in contact with Mazda UK.
NONE of my modifications had anything to do with the fault and not starting.
They disconnected the power train control module (ECU) and checked and fiddled with the connections and put it back together.
An extremely helpful and patient service chap has just rung me and said it’s fixed!
It starts and runs with no fault codes etc etc.
They going to keep it overnight and keep it charged up and recheck tomorrow.
Time will tell of course.
He quoted me initially £75 for fault diagnosis, but then £150 for the extended work.
Personally, I think that’s a bargain and great efficient service.
Just one of life’s little mysteries!
Many thanks once again for all yours and everyone else’s help.
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