Thanks for the detailed explanation, I get the gist of it but definitely need to read up on valve theory, so thanks for the link also.
the grid is connected to HT -ve (chassis) via a resistor
Regarding this bit, my grid has no resistor but is connected to a tuned circuit so is connected to chassis via the coil L2b.
https://1drv.ms/u/s!AlDn_84okuoJr0yDZUOIlbWixKhu