Hi Ollie,
Presumably the charging light worked correctly after you fitted the electronic flasher relay. If so then you can probably discount that as a possible cause.
The first thing is to check the bulb but you could also check the charging by connecting a multi meter across the battery. With a charged battery you should get a reading of 6 – 6.5 volts. Turn on the ignition switch and the voltage will drop a small amount when the points are closed. Now start the engine and the meter should read upwards of 7+ volts indicating that the dynamo is charging. No increase in voltage will indicate a fault in the dynamo, regulator or wiring.
You don’t need a fancy meter, a small digital one will be very cheap and very useful for fault finding once you have got used to it.
Good luck.