Thanks for the replies. To summarise:
Headset Services is either the dog's bollocks or dog sh1t. But they might have an answer.
"Buy a new Dave Clark" - despite the fact I can't afford one right now, and I think I said that originally, and despite the fact I personally don't get on with them.
Plan A is a manual approach - take my headset and a similar one from the pool out to the aircraft - until funds allow a new one. Hey ho.