I suspect that it isn't water getting in as it didn't happen straight away.
There wasn't noticeable sparking and it was only happening on the final spin cycle which is a bit odd. There is a lot of carbon dust everywhere, something tell me that cheap brushes are cheap for a reason, and the motor is noisier than it was, especially in reverse. Do you have any recommendations for brushes?
Oddly I laid it down again, to check the motor wiring and gave the connectors into the controller a bit of a wiggle and on the next try the problem had stopped. It will have a proper run tomorrow to see if the problem comes back. It must be something to do with the controller, maybe a relay controlling the valve solenoid?
I might just buy a water hammer arrestor anyway, just in case!