Took the mistress out the weekend, 2013, 1700 miles with all upgrades, runs for 1/4 mile then cuts out, no warning, no coughing or spluttering, refuses to start for an hour or so then fires and process repeated 1/4 mile down the road. Turns over ok but refuses to fire. Idling and revving in garage it shows charge at 14.3 volts so I think the rectifier is ok. I don't suspect the fuel pump as cut out is instant, Factory suspect ECU and offered me a replacement to try. I do have a problem in that if I do a test drive and having to tow it home again I can't get it up my sloping drive into the garage 😫. Occasionally there's a smell of petrol when attempting to start ( an hour is a long time at the side of the road!) but plugs were dry and looked good. Last time it ran under load it did sound a bit rough, not exactly a misfire though, idling and revving it garage appears ok. Any thoughts Gents?

When it dies, and you go to try to re-start it, do you hear the fuel pump run for a few seconds when you turn the ignition on? I believe it always fires the pump, even if the lines are already pressurized. If you don't hear that - when it doesn't start - it's the pump. Would be my suspicion rather than the ECU.

Faulty connection of the kill switch? Have been threads on this so suggest a quick search

I'd suspect the fuel pump, particularly if the plugs are dry.

It will cut out suddenly if it loses fuel pressure and can restart after a while so that all fits. Check for 58psi at the Schrader valve near the injectors or, just replace with a Walbro anyway as this is a worthwhile upgrade.

The ECU is far less likely as there have been very few reported failures.

My vote is for the fuel pump. Mine did that sort of activity before it died completely.

Fuel pump. There was a M3W on our Cornish trip in March, the engine just stopped with no prior warning it would turn over with a slight hint that it was trying to fire but wouldn't , we changed the fuel pump at the side of the road and it started straight away. I believe it has done over 1000 miles since without a hint of any other issue.

Fuel pump - I had the same problem!

Something I have never done, and would only do with extreme caution ...
To prove its a fuel problem , how about taking the plugs out and squirting a small amount of fuel in each cylinder , put the plugs back and try it.
If it fires , it fuel.
Does this sound reasonable? and issues doing this ?

Ditto on likely fuel pump. As mentioned, when you turn on the key, if you don't hear the hum, that's it. Beware of false positives. If it does start, let it idle. If the pump is bad, it will die again soon enough. Sometimes it takes a few cycles to die completely. Mine failed at 2500 miles in my '17.

I had an intermittent issue like this, got myself a Fuel Injection Gauge Pressure Tester Test Kit from Ebay. Took the guess work out of it. Cost was around £25 at the time
Was the fuel pump BTW
Fuel Gauge