You need one for the brass plate (as opposed to stainless) element, and they have two. I think I'd play it safe and get the thick rubber one.
http://www.orphanespresso.com/OE-Thick- ... _2468.html

Ok great thanks! I'll order it as soon as OE comes back from their winter hiatus.

Ok so I replaced the missing gasket with this part http://espressocare.com/Qstore/Qstore.c ... asket+Flat from Stefano's Espressocare (OE is still closed) and after a day and a half, my thermal fuse blew again. I have noticed there is steam getting under the base while the machine is in use, so I wonder if this could be the cause of the problem? And would that mean the fault lies with the gasket?
Any help would be much appreciated.

Stefano's really helpful in troubleshooting these things. He's very responsive too when you contact him.

Sorry to hear that it's still giving you trouble. I agree with Gary about asking Stefano - he really knows these machines and is very helpful.