Hi Chris,
There seem to be a few different versions of this amp. I seem to have a MK1 without the cutout relay. Finding the correct circuit diagram seems quite tricky (i've yet to find the correct one for my amp
).
If i were you i would replace the main power supply smoothers and output caps as they are getting on a bit now (and are probably drying up and losing capacity).
Other than cleaning dirty switches and generally checking for frying resistors, leave it as is and see how it performs.
This amp has been reported as running in Class A, this is not the case and infact it runs Class B and quite cool (the Drivers run in Class A).
Good Luck.
Cheers,
Alan.