Well I have just been able to do what I wanted to do last Friday & changed the coil. However testing the defective coil showed that is could still provide an adequate output.
So I connected it to a 13.5v power supply & measured the current at 30 sec intervals for 20 mins, periodically testing the insulation at 5,000v.
The initial insulation resistance was 26 Megohms, which is not very good, but it was functioning as a coil. After 15 mins the coil was moderately hot & insulation resistance had dropped to 3.5 Megohms. After 20 mins the coil was hot enough to only be touchable for 10 secs & the insulation resistance had dropped to 1.4 Megohms.
It took nearly an hour before the coil would provide any sort of spark on my tester, despite it externally being only moderately warm. Oh & the spare I had ready had an insulation resistance of 7.0 Gigohms.
From that I have learnt to only break down in a layby & only rely on a coil test once the coil is quite hot.