Stock recovery (Three UK)?

I found my old Desire HD and tried flashing it with the latest Cyanogen Mod through Clockwork Mod. That failed, then I remembered why it has been sitting in a cupboard for three years... because it is S-ON and I couldn't find a way around that. Now when it boots it just displays the Three branding then hangs.

I've tried downloading various RUU's for this device, but they just fail - I assume because of the vendor lock. I tried to find a signed update, but all the links I've found are dead.

I've still got Clockwork Mod, but without a signed update.zip I'm not sure what else I can really do. Any ideas?