So I'd take it to a good shop and have it checked out. Just for reference, the following is what I do when I add yet another tube amp to my collection.

Re-tension or replace loose tube sockets.

Look for cracked or about to be cracked solder joints. There is always a couple.

Check values of resistors and change any that have drifted.

Clean or replace pots and jacks.

Check all coupling caps for leaking DC. I usually change the output stage coupling caps anyways, as DC leak here can cause output tubes to melt down.

Check and replace power supply filter caps if needed, including bias filter caps.

Check and/or replace all small value electrolytic caps.

Finally, check all the voltages around tubes, set bias.

Plug in and rock on away.

Alternatively, sneak it into storage so Mrs. doesn't find out that you bought yet another old tube amp.

I've actually found the culprit I think, one of the plates/cathodes(?) in the output valve/tube had come loose, and was floating about freely inside the glass envelope!!! Is this a common fault?

Anyway I think I'll get a new pair of output valves and get an engineer to bias it properly and to give it a pat on the head. Thanks for your help