[Catching up, so late to the table...]
Hi Gareth, and welcome to the Thorn Cycling Forum!
All congratulations on your new-to-you RST -- what a terrific photo of a lovely bike!
As for the spontaneous gear-changing, I'd continue to ride the bike a bit and see if it happens again. If it does, then -- yes! -- Andy's suggestion of a cable adjustment or Fred's suggestion to replace cables and housing would be spot-on to address the problem.
Maybe, though, it was a one-time anomaly and won't repeat (fingers crossed for luck).
In any case, a wonderful ride and so glad you have it; may you enjoy years of happy miles riding together. If you run into any problems, give a shout; lots of helpful advice here.
Best,
Dan.