The particle filter was becoming lost when the robot performed a turn - this ultimately turned out to be a problem with the particle filter resampling interval, but it opened my eyes to the fact that tracked vehicles do not perform deterministic turns.

Observation revealed that when one track was stopped and the other moving, the pivot point would not be around the center of the stationary track but rather around one of the ends of the stationary track leaving the dead reconnecting very wrong. The change in heading was off by almost 50%.

I had previously stopped using the compass in the dead reconning process because of the presence of large magnetic fields like the fridge etc. The lsm303 compass also seems to crash regularly (suspect I should by a new one). One day I will install two compasses at opposite sides of the robot so that the presence of local magnetic fields can ge detected, as a local magnetic field will cause the two compasses to point in defferent directions.

The plan now is to use a gyro to feed change in heading data to the particle filter.