I now have one channel working quite well now. Still a little bit of RF, but it's compressing properly.
The meter circuit also is working, although I can't get it to zero when there is no GR happening. This could well be because I am not using the specified meters. So, I will probably need to play with some resistors to get it working.
One thing worth noting is that if turning the output pot up a lot, the a sine wave seems to distort quite badly, which made me think that the unit had an issue. So it seems quite level dependent. This may not be an issue with an original unit where these controls are ganged together. However it could be a transistor biasing issue, where some of the resistors have been changed from the original values.
SR1200 do you have a way to check these observations ? I would be interested to know whether it's just me getting these issues, so I know whether I need to carry on tweaking the circuit.