First suspect is the rectifier.
Can you measure the voltages over the 5 or 6 settings with a multi meter?
DC voltage set to the 200 range, between torch tip and return lead, it should be around 20 to 32 volts.
If too low, I would then check the rectifier diodes all work, using the diode function on your meter. You might even see the visible damage with some of the wires detached.
If they are, you can't fix them, so need a replacement rectifier.
Sealey's spare parts service are helpful, and often cheaper than Ebay. This model isn't listed any longer online, so phone them.
Your rectifier will look similar to this. It may have 3 or 4 plates.