Andy,
Not tried the resistor fix yet, been busy and don't know when I'll get the chance. I'm in no great hurry yet as the 'box is only mildly recalcitrant at the moment and is quite a way down my list of priorities. I need to clean the TPS as well in case it's sending dubious messages to the 'box. This could possibly be the root cause of my problems.
Anyway, the car is currently sat powerless as I recharge the battery which was as flat as a particularly flat pancake yesterday.
But I would still like to know of a good, reliable autobox specialist just in case the 'poo hits the propeller' some time.
A big gearbox specialist down here in my neck of the woods is TES Transmissions Ltd in Westbury. Does anyone know anything about them at all?