Just had one of these pass through my hands for repair, it was the SECR liveried R3648 - which seem to command quite a premium on ebay, if you can find a buyer.
The motor was playing up - stoppong/starting and smoking quite badly after stalling. Suggested over lubrication, but the owner confirmed that he had never been near it with the 3in1!
Opening it up, I found the gear train had been basted with the white grease (silicone) that Hornby use during manufacture.
So much grease had been used it had found its way into the motor and onto the brushes and commutator. Judicious removel of the excess, followed by a slosh of IPA and an extended period of running in saw the motor back to smooth running and a relieved owner.