I've had exactly the same problem -- 3 times now.
Last time was just over 12 months of purchase and after a bit of a battle with Countax they authorised the dealer to fit a replacement (may 2009).
They say the oil should be replaced every year -- but the dealer says no need!
I've previously had 3 Westwood (man gearbox) tractors with no such problems -- yet Countax say the Tuff Torque they fit is stronger than the manual!
DJW
April 2010