Had a quick scan on Ebay to see if I could get similar / larger brushes and file them to size. Cheapest similar ish ones were £12 plus all the faffing about so I went to these guys
A.J.W. / A.C.W. Electrics | Car Electrics - Yell
For £15 they're supplying and fitting the new brushes (filed down to fit and soldered in properly), cleaning and reasembling the motor. All in 24 hour turnaround.
Used them before for Mrs's car starter motor repair and can't fault them, really helpful and really cheap!