Thanks again for the replies. I sent an email to Williams asking for price etc for them to collimate and I'll await their reply. That was interesting, Tony, as mine was bought ~ 2008/2009, before Williams came out with their digital focuser. I can't check more accurately as I disposed of all the images from that time because of poor quality.
I tested the centring with my Glatter laser and the dot (on the wall) was within 1 - 2 mm of the centre so it can be improved a little but I doubt if it would cause my problem. I rotated the OTA to check this but of course, if the tube is not round, the test wouldn't be very accurate.
Charles