So it
was a case of a missing adapter and no-one had ever seen what I needed. Decided that it was time to create my own solution, so cut off the two ISO plugs on the car harness and also the supplied (wrong) adapter on the head unit harness and soldered everything to make a decent job and...... it works.
I think I just needed to calm down and think outside the box a little. Thanks for the input.
Dougie.