Good News! After leaving the above mentioned 12.8mm diameter socket overnight it was slightly wider but I still could not get a router bit in there. So, I tried again with a slightly wider socket. This time I could insert a bit about three quarters of the way through before it would jam up so I also wedged a small screwdriver in the slit of the collet to stretch it open a little. . It's seems to have done the trick without causing any damage. I can now easily insert and tighten bits and have used several today just to test it out.
Thank you for all the above contributions to this thread.
Cheers
Alan
