Hi All, Thanks for your replies,
I have 3 workshop manuals which state a 13lb cap is correct. After a short journey it has pushed out the coolant below the radiator tubes when I remove the cap. There is no sign of a drip or any external leaks at all. Someone has suggested mounting the radiator higher to remove the air lock issue. Another suggested a bigger radiator that goes right across the front, rather than the one with wings that I have.
Mr Pete Lewis however has hit the nail so to speak, I measured the radiator cap depth at 18-19mm the neck is 20-21mm EUREKA. I am ordering a cap now from Rimmer Brothers.