Andy, Iain, that is brilliant, that all makes perfect sense, thankyou! I agree with you about the transformer.
This has given me another idea though... The iPhone uses a 3 ring plus tip mini jack, (ground, left, right and mic). In theory, I recon it would be possible to get an iPhone cable with 3 ring plus tip mini jack, hack it to pieces and wire it to a Clansman 7 pin plug, thus allowing the mic in the headset to be fed into the phone for calls and obviously audio from the phone sent into the headsets. I think iPhone mic on A and audio on D and E with ground on G.
Downside would be that everyone on the vehicle harness could listen into and talk into the conversation!
What are your thoughts?
Thanks again!
Dunc