My trusty rear diff ruined my night tonight at club. In round 3 it felt solid and was difficult to drive for the 1st minute of the race which lost me time. after a minute it loosened up so I picked up the pace regaining time.
After round 3 I stripped the diff down and found the balls and rings dry and no grease in there. cleaned and re-greased and rebuilt it. Felt nice so I put in a minute of track time in between races and it came loose. tightened it back up and reinstalled for the final and within a couple of laps it came undone.
Slightly against any BRCA rules I swapped the diff mid-race and carried on (It was a club night so didn't matter), but the car wasn't as good as when I had the original diff in.
New washers and C clip on order tomorrow I think