Just to confirm - took it along to Lancaster's yesterday - was worried that they wouldn't be able to spot the problem as it only happened once on the way up there (80 miles). But one of the guys there took it out for a spin and instantly found the problem. It is now booked in to have a new 5th gear.
To test, put it in 5th at a relatively low speed (30-40) and accelerate hard for a short while, then lift off and accelerate again... drops straight into neutral if there is a problem.
Cheers,
James.