Thanks for all replies.

I have successfully re-sized one of the cases, but only by taking my R.C.B.S. full length sizing die down to about as far as the thread will go.

As I said initially, I have re-sized many once fired cases and perhaps about a half turn decrease was required, what rifle chamber this came out of god only knows!

I thought that it must be something else that was causing the problem.

Hopefully I will be able to re-size successfully the rest of the batch, one thing with buying "once fired" cases etc is that you need to run every sized case through your rifle chamber to make sure all is ok or risk a possible jammed round at the range.