The transom had cracked and split between the glassfibre transom rear and the internal reinforcing on the knee side right along the top edge of the transom.
I am told it wasn't any more than a few inches deep but it wasn't clear until it was ground back for reglassing.
They did it gratis, if you can call three days travelling and £350 in fuel and expenses gratis!
but I was told the lightpole on the transom had caused it and warned that it had to be reinforced or removed in case it happened again.
I may not be a fibreglasser but I am an engineer and do understand stresses and don't really accept that explanation. However they were good enough to repair it and I am going to cut down the pole 14" and add a brace to the deck just in case I have further problems to save any arguments if it happens again.
It was ground back and reglassed and looks like new, time will tell it is as strong as before.