In the past I've seen people allowed onto See coaches about to depart to other locations than the one they've booked (just to fill the coach) - but only those with a see ticket in the first place.
Seemed more regimented and strict last year though - and for coaches to Manchester we were clambering to get on even though we had tickets as many coaches hadn't turned up on time (this was about 9am Monday). If you have to be in Leeds I would go with this suggestion and book the National Express.
All the best, I鈥檓 sure when the pains subside and the ops are over you will feel the benefits of the procedures. Your last sentence is so true. As the Specials sang. Enjoy yourself, it鈥檚 later than you think!