I have found one of 8 hours 8 mins - Coombe Junction Halt to Melksham. It does include a wait of a few hours at Cheltenham Spa however:
Depart Coombe Junction Halt 10:52, arrive at Liskeard 11:03.
Depart Liskeard 11:09, arrive at Cheltenham Spa 14:40.
Depart Cheltenham Spa 17:40, arrive at Melksham 19:10.
Fair go, but your journey includes the evil
XC▸ , is not a permitted route, and requires multiple tickets.
Opps, didn't realise it included XC