[Arran]

That is an interesting arrangement, the field coil is parallel connected with the bias resistors on the negative side, in most cases, should they wire the field coil on the negative side it would be in series with the bias resistors, somewhere between the center tap and the chassis. I also noticed that the parts list specs most of the carbon resistors as 1/4 Watt, so you may want to check those over as you replace the paper caps, 1/4 Watts are more likely to drift upward in value then 1/2 Watt resistors, or go open, particularly if used as a plate load resistor such as R10, 220K Ohm. I would also check the bias resistors, R12 and R13, 1/4 Watt rating sounds way too low for those even if they are in parallel with the field coil, in most sets those would be 5 Watts at least.
Regards
Arran