if you've ever met a Master Driver and been wowed by how spectacularly dull and jobsworthy they are, they are in good company with the SASC. MDs are normally also the blokes who didn't do Sergeant Major's jobs but went the 'technical route' instead; can't comment on the SASC WOs i've met, never managed to stay awake long enough for an explanation!
The SASC are specialist Corps of Sergeants and above who traditionally specialised in the delivery of Skill At Arms and Fieldcraft although they are now involved on all aspects of training including design, deliver and validation as well as a myriad of other areas (coaching, advisory)linked to training for operations. There is no direct entry so you have to join up with another capbadge (Infantry, RLC etc) and then get recommended either on a course report or via your annual report, pass their selection at Breacon and then you are re-badged SASC as a Sergeant. Hope that's useful.