The only way to guarantee a HST is to get a Leeds service.
In theory you can tell by looking at the EMT TT as it is (or was!) colour coded, but a substitution could occur to a train that terminates at Sheffield.
In the morning that is going to be a bit more tricky as the tickets for peak time services sell out fast!! But on the way back it's a bit easier. I know you said you want a fare from SHF, but a tip: if you book to/from Leeds (specify via Leicester) you can be sure the train goes to Leeds, and it does not cost extra to do this (as far as I can see). If you want a ticket from Manchester, specify via Hathersage and it should offer AP fares to Manchester. (You can still get a TPE, as it will not reserve you between MAN and SHF).
I would not wait, I'd log on to a booking site now (
NXEC's is good, if you like thetrainline interface then try a site like
FSR as they don't require you to log on before searching) and get those fares ASAP as they've already been on sale for a while and many of the best fares are already sold out. Only the 21:25 from STP has the cheapest fares available at £15 FC. And going from SHF to STP you can't leave until 11:18 at that price!!
