I have a Joe Meek C2.
I’ve lost my power supply but as I remember it was an AC to AC 12volt 300 ma.
The nearest I can get to this in a generic supply (which came with it and worked fine) is 500ma.
Would this be OK as there is a 200ma difference.?
Yes, using a higher current rating power supply is fine for most any gear. The unit only draws what current it needs from it (ex: if the unit needs 300ma then it only draws 300ma from the 500ma supply…and the extra 200ma in current doesnt get used by the unit, but is there in reserve if needed). Just don’t go lower in current, and of course for the Voltage rating, keep that the same.