This thread reminded me that I have one of the affected models (R2413) and dug it out. After chipping it ran a few times, but it hasn't been used for around two years. Sure enough, it has the same problem.
This is the worst corner. As with others, when I tried to dismantle the model to recover its decoder one end broke away and it's virtually impossible to get it all back together again. However helpful Hornby are, this one is beyond repair and will need to be written off. One phone call to Kent coming up methinks.
By the way, I also have the R2649 variant (31 165) and this is fine.