Hi people, problem solved.
Tried everything in the book for several hours and nothing worked.
As a last resort, I found the original usb cable supplied with the phone and compared it to the one I was using. It’s the same apart from the plastic surround which is a little sleeker than most other usb cables, including an old htc cable I was using.
Obviously the surroundings of many other usb cables seem to fit (as mine did), but don’t quite lock in perfectly causing problems.
The other cable let the phone charge, but wouldn't connect as it's a mil short and is stopped by the galaxies casing
The samsung cable fits perfectly and my pc immediately recongises it.
Amazing the problems an extra millimetre of plastic causes. Duh !!