In reply to TobyA:
> I wouldn't mind popping into Decathlon either so maybe two birds etc.
If you're confident you'll be under 2 hours you could park
at Decathlon (for nowt).
But they have cameras on the entrance and exit hooked up to an automated number-plate recognition system, so if you go a minute over 2hrs you will get a "Parking Charge Notice" (see UKC discussions/debates passim, ad nauseum, on how to deal with those).
As above - parking is surprisingly easy and reasonably cheap in Sheff anyway. (Certainly compared to either Nottingham or Leeds) Just avoid the shiny new multi-stories and it won't cost you an arm and a leg.