If you know the specific dates, you can book two singles for £9.50 each way. However, you must travel on specific trains on those dates, otherwise you'll have to buy a whole new ticket.
If you have a YP railcard, it makes no difference to the fare above, but you can get a walk on Saver return London-Leeds (use on the day, return in one month, some restrictions on peak trains)for £47.25. However, if you split your ticket to Leeds (train must stop at Peterborough, most do), you can do Cheap Day Return London - Peterborough for £11.95 (£14.70 on some trains) and Peterborough - Leeds £29.50 Saver Return.
Might be some cheaper options available, I'll have a look. Don't know much about the National Express apart from probably be cheaper on that.
I managed to get it down to £14 (funfare) for a London-Leeds return on the National Express coach. That seems pretty damn cheap to me.
It makes a big difference in prices if you:
- know the dates you want to travel and can book in advance
- want one way or return
- if return, want to return same day or within a week/month
- carry discount cards such as a Young Person's Railcard or Discount Coachcard.