Thought I'd bump this just to say its now fixed!
Took it to the local garage in the hope that I could avoid a trip to a dealer, and the guy there sorted it. Apparently, it was a failed switch on the gearbox somewhere that wasn't telling the brake computer the car was in a forward gear. £20 for a new switch, £50 for diagnosis/removal/fitting. Well pleased!
As a matter of interest, I was also getting a dashboard message telling me to 'apply footbrake' before the start button would work. Weird.