Here's my progress this evening. I soldered in the wire bypass, I took the inner cable from some three-core mains plug wire which I'm sure will suffice. I was able to get a bit of solder to stick the broken part, just for good measure and then covered that with some hot glue.
Also, all four new brushes are in, two of them were fine for a bit more use but in for a penny, in for a pound!
The four brushes seem to be wired like this:
1 - Black
2 - Red
3 - Purple
4 - Connected to brush 2
It was 1 and 4 that were the most worn. Make of that what you will!
Hopefully I'll get time tomorrow evening to put the motor back together and see if it's all working.