Thanks for all the great feedback. I've tried all the options on the check list with no luck.
I do have few observations.
1 - a pair of headphones plugged into the amp work fine with both channels working fine and the balance control smoothly fading from one side to trhe other.
2 - the amp has two sets of speaker outputs and the problem is present on the left channel of both sets
3. - the fuses to both speaker sets look fine
4 - whilst the balance control works fine on headphones, it makes no difference to the volume on the one working speaker when faded from side to side.
I guess my next step is to open it up and try the contact spray.