Thanks for the heads up on this John.
I contacted Vitesse this morning and as the person I needed to talk to was out of the office it was suggested that I send an email. I therefore sent an email with a reasonably detailed description and this evening I received a reply. Based on my description the most probable cause has been identified a being in the remote extension housing on top of the gearbox. This can be removed by removing four bolts and can be replaced on a exchange basis.
It seems a similar problem has occured on one occasion in the past and construction methods have been changed to ensure it does not happen in the future. Most likely my gearbox is a fairly early unit that was built before the changes were implemented.
All I have to do now is remove the gearbox cover, something that I hope I can do without too much difficulty.
Vitesse have proven themselves to be very serious, correct and prompt in their after sales service and support. A rarity in this day and age which is to be applauded.