Looks like this is not perhaps a common fault or at leat noone come across it, but i will update this thread as i work through it
Today i finally got the driveshaft off and found the abs ring was indeed all but gone - only an inch remained but that crumbled away soon as i touched it
New ring fitted easily with threadlock applied from reluctorrings.com so just to reassemble tomorrow - had to take off shock, control arms and trailing arm to get driveshaft out. I marked the lower arm but realignment will be needed
So just hope now i put it all back and problem solved, will update as i say in case it helps someone else in future - even if it does not solve it.