East Midlands Trains via Sheffield i would have thought - im £20 each way which isnt too bad.
For all those coming up via London, check MegaTrain.co.uk. I've used it home from Uni every so often, and you can get singles to and from london for £10 ish (often less) without any form of railcard. Standard advance with a railcard is about 12/13 quid each way. Book early though or you'll be looking at £60 +