Hello again
Thanks to everyone for their input.
We're presently speaking with NWAS about this but there is concern from them that most of the available 12v current used to power any on-board equipment is pretty much spoken for. Tales of having to leave the vehicle's engine running all the time even when parked, just to keep all the electrical/electronic systems running are quite common.
So, have purchased a Mascot 300w sine wave inverter from RS 494-5446 @ £166 for evaluation.
I'll report back as and when.
Cheers