Well, after a couple of days of tactical messaging/phoning/emailing I believe I've got the best outcome.
He's agreed to pay me £300 as a final settlement, and we move on and get on with our lives. By rights I had to give him every opportunity to fix the car, but I most certainly didn't want that to happen as I believe he would bodge it, Again.
I said I wanted to witness the car in a dismantled state so that I could see the condition of the head/gasket etc or, equally I would be happy for an indipendant garage to carry out the work so long as he contributed £300 towards the cost. Result.
Thanks again to all of you for your input, and John&Sue....great idea about forwarding the dealer review site (s)..... I reckon that may have had an impact.
All I need now is for Russell to get in touch......