Further update:
Tested again this morning. Garage below 5 degrees C, but full battery from yesterday's charge.
Immobiliser disarmed in two seconds. So........I'm thinking that this must be battery voltage related. The sensor coil is permanently live, as it has to read the key before the ignition is switched on. My instinct tells me this shouldn't be 12volts, but I haven't tested it. Clearly there's a very tight margin around 12volts whereby any reduction de-sensitises the coil.
I don't see an easy fix to this, other than a fully charged battery.
Maybe remove or bypass the Morgan fitted immobiliser and use an alternative method of immobilising the car. That's what I did when I removed the old ECU.
Not sure how easy that is on the more modern cars but one of our electronic gurus should be able to find an answer.
Plenty of stand alone immobilisers on the market.
