[Boater Sam]

Hi Al.
The valve bases are good, the paxolin one is spotless, the others are ceramic and clean. I had considered anode to grid conductance but can find no tracking and the pins are not close. I had tried more than one replacement 'pull' EBF80, the howl was exactly the same. There was no switch cleaner used on the bases, I have found that this can cause tracking particularly if it has not fully evaporated before applying power.
The fact that I could not cause any alteration in the howl no matter what I did lead me to consider that the feedback had to be across one stage, dropping the mains voltage with the lamp limiter was the only thing that changed the pitch.

The "aerial" leaves a lot to be desired, originally it was a bit of wire up from over the bench to the floor joists above, draped across the room at right angles to the bench, about 4m of horizontal. The workshop is the old corn mill room with the loft above, very old.
The ceiling got underdrawn with OSB, hiding the wire. It has had another down tail twisted on to the workshop set, far from ideal, but it has never caused any pickup problem before. The racket on the mains from the fluorescent light, the lights in adjacent units and all the milking parlour computers and pumps is bad but to run a wire outside the other way would be impossible, the silage trailers that use the track are over 14ft tall, more when they drive out with the rams up.

I am looking forward to the next AC41, if its the same problem I would be amazed, Bush must of sold thousands of these.

Thanks everyone for the input, much appreciated.