Are the HSTs to the Cotswolds often busy then? They always seem empty to me, but then again I've only been on weekends.
Yes. They don't run HSTs in the weekday peaks and on Sunday afternoon services back to London for the fun of it. The 06.42 from Hereford can be full and standing arriving in Oxford in the morning and the 17.22 from Paddington is also very heavily loaded leaving Oxford.
Off-peak HSTs are quiet, but they are run either because that's what stock is available (in the middle of the day), to get the trains to the right places for the next morning's peak services, or to form Oxford-London afternoon peak fast trains (the 16.01. 16.31. 17.01, 17.31 and 18.02 departures from Oxford all come off the Cotswold Line).
Weekday HSTs in the middle of the day look likely to be among early candidates for IET conversion, maybe even at the same time as IETs replace 180s at the start of next year.

