[Arran]

L5 is the field coil of the speaker, if it was open I don't know why pin 8, which is the cathode, would would have a lower voltage, if anything it would read high since there is a filter cap there and there is no load. Check the voltage on pin 4 of the 6V6 tube, it should read 230 vdc, if it reads zero then the field coil on the speaker is open. Now the speaker field may be burned out or it may be because one of the leads has broken off internally. If it has burned out then it's very likley that C18B or C16 is shorted or near shorted.
Regards
Arran