It might be helpfull Graeme If I let others with a GSO Dob know , to check there focuser alignment with a laser as we did with yours.
We had to space Graemes focuser on one side as the laser was pointing off to one side of the diagonal by about 15mm . I am not talking about didgonal off set either . This is towards the side of the tube not towards the mirror . Spacing was not a problem but the play in the focuser could not be removed .
The GSO focuser would be fine for a longer focal ratio scope but for an f5 or less you really need to have better accuracy in the set up and draw tube .
Your newly replaced mirror looks a lot better with out that cat urine mark on the surface .
Also nice NOT to see cleaning scratches on it where a dealer tried to clean it .
Mark