I can see four possible codes relating to P0615 in the service manual:
All have the same description “Starter relay circuit malfunction” and warning: “flashing push button start indicator light (amber)” but only one has a fail-safe, P0615:12:
I think the first step is to determine if the Starter Relay is the culprit, this video gives a suggestion for that:
Hopefully we can get you back on the road soon!
Just re-read your post and realized I can’t read, you said type 12 in your post! 
Excellent, thanks again 
Right, Update:
You won’t believe this.
So about 6 months ago I had a new windscreen fitted because I had a gigantic crack in it.
That windscreen replacement allowed water to drip slowly into that C-016 connector (I think), related to the PCM and Start/stop system, which has corroded it.
This isn’t a poor replacement windscreen fitting, but an actual recall/problem that Mazda are aware of apparently.
Be careful out there and check for corroded connectors.
Shout out to @Roadster_Robbie for being an absolute legend and spot on.
Cheers
The issue persists regardless…
Have you had a chance to try a different starter relay?
Been popping in here from time to time and have to say I admire your patience and the way fellow owners have stepped in to help you out, wish I could be of more than moral support but such is life. Hope your car makes a full, speedy and above all lasting recovery.
@Art_Tidesco appreciate you popping in and leaving a message. It certainly is testing my patience, but the car has been great for the best part of 4 years and has been (mechanically) bulletproof, so I can’t fault it.
Whilst the fault continues to elude both the mechanics and I, have to say, I’m learning a great deal about my car and learning how useful breakers yards can be for parts!
Cheers
Update: We have gone through the wiring loom, the start-stop unit, the battery, fuses, relays etc. All okay.
Current working theory is the clutch position sensor is malfunctioning, will see tomorrow.
Will update you all accordingly.
Cheers
Edit: Forgot to say, the ECU is throwing P0615: 12 & 13 errors now
So, local garage has given up after 10 hours of looking into the problem, no idea what’s wrong. The car is now with Beechwood Mazda Derby.
Cheers
An exhausted MX5 owner.
Faulty ignition switch? - Not sure if your model has “keyless go” but i had a vaguely similiar issue on my CX3 with keyless
Faulty CPS (crank position sensor) - unlikely as that manifests itself as a hot start issue where the engine turns over but doesnt fire
Does depressing the clutch make any difference?
An issue with the immobiliser? If this issue started after a new window regulator and motor perhaps some wiring was dislodged?
The front end damage may have triggered something - the “active bonnet” senor may be faulty?
All good suggestions Blue Bird.
Could be the ignition switch but clearing the code clears the behaviour.
Depressing the clutch makes no difference, could be a faulty clutch cut off switch, but again, clearing the code clears the behaviour.
Wiring loom has been checked and door has been taken apart twice now looking for that issue, no dice.
Active bonnet could be a contributing factor, but the behaviour manifested before it got to the garage and was hit by the other car.
Will let you know how it goes!
Cheers
it isn’t clear, to me, if the issue is the car wont turn over (likely electrical) or turns over but wont start (mechanical / fuel).
It sounds like both of those have been investigated but if clearing the codes fixes it, but the error and code returns, does that point to that component?
The car wont turn over, refuses to get to the “ignition” stage. Fault is almost certainly electrical.
Problem is, the error code points to fuses and relays, the PCM, the battery, wiring and the start stop unit.
But that’s all been checked. 
Cheers
I soent a winter trying to work out what was wrong with my Ducati 999.
It turned out to be the fuel pump cable that had fractured inside the insulation, so it looked intact but only worked intermittently.
So do check your cables for continuity.
Hello, sorry for the silence.
Beechwood haven’t been able to replicate the error and have said they don’t know what’s wrong.
I’ve given them the fault codes and full history from the past few weeks.
Might get to speak to the ‘Master Technician’ next week…
Cheers
interesting reading, glad i stopped at the 2.5 model ownership. both mk1’s i have had had wiring issues, thankfully mild compared to on here. one with a new battery blew the 80 amp fuse second time i tried to start it after fitting this new battery a Yuasa 44AMP, a faulty new battery, and still havent found the parasitic drain flats after 4 days,just remove the pos lead when i park it ,other with a dash issue was a broken wire took days to trace it and run a seperate wire outside the loom… in some of the cases i read on the issues here i would be sorely tempted to allow them catch alight and claim the insurance. the modern vehicle with so much totally unnecessary electronics running cambus through an ECU often sited in totally the wrong place listening to conversations amongst modern auto tek guys that i’m in touch with they now know that the electronics are more important for all the wrong reasons and the normal engine gearbox brakes DMflywheels clutch while today severelytightly packaged are still quite reliable in good hands ownership wise…and most recovery guys that i know say most of their daily recovery work is on modern stuff with electronic issues…so the price of all the extra ‘‘goodies’’ in a neat and loved by many sports car is headaches when they go awry…w
@wacee I am beginning to feel this way, the car is mechanically sound but for some reason, won’t start.
Garage called, it hasn’t faulted in over a week now so it’s just sat in their car park working, which is irritating.
They have checked over the connectors and fuses etc etc, nothing wrong with it at all.
I think I’m going to leave the car with Beechwood for another week, see if it faults. Maybe all this pulling connectors apart, may have solved it?
Do you live near a radio transmitter or mobile phone mast?
@anon35904086 Not that I am aware of.
Also, car refuses to fault, so i’m going to pick it up on Wednesday and pray…I guess.
Cheers