As a footnote, if you are buying the ticket on-board the train at Montpelier and the guard cannot find the ticket, it is listed as "FOSS▸ ROVER 8IN15" under the "Rover Enquiry" screen.
Just in case anyone is searching this forum, the even simpler code for this Rover ticket is "WR7"
I've saved ^ since hearing about it on this forum - still encounter
FGW▸ ticket sellers who haven't heard of it!