This brought back memories of Weymouth Beach on Sunday afternoons listening the Light Programme between swimming.
I replaced R11 as the 4.7M resistor was over 12M and causing severe distortion. I am now fitting re chargeable cells to the battery packs allowing the use of a charging system. The heaters draw 120mA at 1.2volts so a 1.2v NiMh cell is the answer. As for the 90v pack, 10 PP3s in series will be the answer.