Having spent hours trying to prime and bleed the system the 12 mm pressure release nut has developed a weeping leak.
Is there and easy fix, can I swap it for the one on my other soon to be scrapped xm without the dreaded ballbearing fallout?
Thanks in advance.

If I recall, there is a simple standard oil sealing "O" ring on the bolt. With care, you might manage to do the job without losing the ball, but I would suggest a suitable sheet underneath to make sure. It is a standard-size ball, but don't ask me what size. All PRV's appear to be the same - after all, why should they be different?

Remember PRV balls are available from Citroen dealer for...............4 pence.