[Arran]

Post war battery sets like this one don't normally use "C" batteries, in fact very few 1.5 volts sets do, they use resistors for bias instead, this model included. So if there is a bias problem it's likely a bad resistor, or one with an open connection. It does however use loctal type battery tubes that use short pins, sometimes the pints or the sockets can develop oxidation that leads to poor contact so it's worth checking that as well.
Regards
Arran