The Respirator is designated the CT12
The newer helmet used by the sas is indeed nickanmed the defender however most troopers still prefer and will wear the older type AC100/1 composite helmet (the one you wil see in most SAS CT pictures for all you book readers) The defenders main improvment is that it adds protection over the full CT100E electronic ear defender headset as opposed to the AC100/1 helmet that only partially covered it.
There are loads of AC100 helmets hitting the shelves in army stores and markets now as they are being heavily phased out so the CT fellas arent going to have much choice in what they wear
I believe my details have been passed onto you already, if you do need any advice with your SASATT guys just get in touch mate.
Heres a link to the company that make the helmets, maybe a nice e-mail to the mwill provide you with some ref pics - if not, again - just get in touch
a shot of the AC100/1