I don't know for definite, but at that time of day, it's almost always a Class 222. The xx:49 are the slower services to London so are generally 5 coach units, although I've read on here somewhere that there's a 4 car diagram as well. From living in Sheffield for uni since September and having to walk past the station to commute from my flat to uni, I would say that first class can be at any end of the train. I'm think they do prefer it to be at the London end, but I've seen units turn up with first class at the north end fairly regularly.