After various attempts by "experts" and Renault, I was getting nowhere with this problem and it was getting more frequent, so I decided to have the immobiliser "immobilised".
I contacted Remote Key in Birmingham, Remote Key Fob Replacement
who I saw mentioned in one of the threads on here. I took the car to them and they modified it in an hour while I had lunch at the Asda store opposite.
This was 2 weeks ago, since then no problems at all. When the immobiliser fails now, the car starts OK but the LED still flashes on the instrument panel. I can live with this as you soon dont notice it. Everything else works fine, remote central locking etc and the usual LEDS flash when the car is locked, so giving the appearance that the car is alarmed.
Good value I think for £100. They tell me they are going to be trade only soon, so get in quick!
Thanks to all the help from this great Forum and to Remote Key for putting me out of my misery