Thanks for the response.
As I wrote near the end of my email, I have never been able to find a serial number despite having the chassis out and looking really closely all over. I can send photographs if that would help?
I just tried the test you suggested. About three times the reverb appeared to have cut out so I stopped playing and rocked the cabinet. I got a mild crashing sound but as soon as I played a note on the guitar again the reverb was working again. Then for the last 20 minutes of testing the reverb didn't cut out again. It's unpredictable like that when I'm testing it at home. Not a definitive answer I'm afraid.
Although the method seemed sensible, I couldn't get the amp to drop the reverb signal to order or to make a crashing noise in a predictable way that would give an idea of what was wrong.
If the act of shaking the amp revitalised whatever was causing the reverb dropout, I'd hear the crashing always (although I'd tried gently banging the amp before when the reverb failed, with mixed results).
I reported this back to Bruce. His diagnosis was incisive - see part 15.