Thank you everyone for your replies.

Well, since this message was posted, I noticed arcing from a part of
the tube bowl (top-right corner) where glass has been knocked out to
the dag, with a long crack emanating from this damage right to the seal
between tube bowl and front glass screen.

The arcing at this point only happens once every 10 or so switch-ons,
so I hadn't seen it before.

I assume air has got in and the arcing is because there's now no
electron beam flow from the cathodes due to air resistance and
impedance and so the EHT, with no electron beam current to sate it, is
seeking a point of lower potential and finding it in the form of the
grids and cathodes. This is only a theory though, since I have not come
across a CRT with air in it before. Anyone know for sure that this
effect would occur if the tube was absolutely full of air?

Thanks again for your kind help.

Best regards to all,
James