Thanks Brian,
I agree that focus must the problem, or at least part of it.
Both the mirror lock and the focus mechanism seem to move freely / as I would expect, and don't seem to be disconnected or damaged.
I haven't checked from the front of the OTA, but when I look down the diagonal without the eyepiece, I can see the image clearly, I can see it moving in and out of focus and getting smaller and larger as it does so.
As to whether the primary mirror is actually moving sufficiently to achieve this I am not sure and further investigations will have to wait until I get home.
Thanks again for the thoughts
Neil