[Boater Sam]

Taking all the previous comments on board, had a thorough dig into this set this afternoon.
Removed and tested every valve, cleaned all the pins, cleaned and inspected the sockets and checked that the IF amp valve socket had a grip on every pin.
Checked all valve voltages, close enough to spec on the Avo 8.
Released and re-tightened all ground tag bolts and nuts, went over soldered joints with a magnifying glass.
Shorted out the extra grid stopper on the IF valve, the howling immediately started as before. Dabbed decoupling 0.1uf caps all over, even cutting out the signal, the crazy thing still howls.
Set it up with a tuner running into the gram sockets, clean audio at good volume.
Scoped the anodes to find that the EBF80 is oscillating whenever it had HT no matter what I did with the volume control or the wavechange switch or decouplers.
Rechecked all components around the first 2 valves, checked the IF transformers had not been twiddled. I can find nothing faulty in this set.

So, like it or not, this set is finished and the 8k2 grid stopper is staying in as it now works just fine, even short wave is working really well.

But the story goes on. I have just dug out another AC41 from the shrubbery to compare the chassis, and this one is oscillating too! The symptoms are very similar though not so severe.

We shall see how this one responds, next week. Watch this space please.