Thanks all, will put a scope on it tomorrow - I did hear a squeal from one of the 2N3055s before it short circuited.
From what I can see at the moment, I should be well within the operating area (the power supply current limits at 4 Amps, or 2 if I set it to that) - see attachment.
The reason why I put the 0.5 ohm resistor on the collector was so that there was no chance that Veb could exceed 7 volts (abs max specs). It's late, so I not be thinking at the moment very clearly - if the transistor turned on with 2V at the base so that the 20V drops across the 0.5 ohms, would I not exceed the 7V limit? As I write this, it seems wrong, because the transistor would not turn on in that state...
I've attached the datasheet to this post if it helps.
I bought the transistors from ebay, so this may be the issue, but even if I limit the current to 2 amps so that they don't get fried, even when I put a lighter on the thermistor, it does not seem to change the current through the transistor by much.