Progress update..
Took the opened motor to another motor specialist today to ask about the carbon brushes as it looked like they were at the end of their spring travel.
He removed the bearings and we were able to remove the round plastic with the carbon bearings and copper braid connected to them.
Surprisingly the carbon brushes still had a lot more travel on them so they're not the problem. The real problem with these motors is the commutator - the bit that spins around inside the brushes attached to the motor shaft. It's worn right down by around 1-1.5mm. So much so there's barely any copper left under the grey mess.
The guy i spoke with said the brushes can be replaced for approximately £10 per piece but as there was still some travel left in them he didn't think it worth worrying about.
Inner side of the two bearings
These next couple of pics taken in the car while waiting to pic my son up... As you can see there is a copper rim left on 3 of the commutator contacts
When we removed the bearing and got to the commutator, the 4th copper rim was loose - flapping about. I suspect it is these that break off as the commutator wears down and they they jam between the commutator and the carbon brushes. This might explain why when left for a few hours (the motor can relax) the bit of metal can then move slightly and then the motor can spin up the next time.
To show you just how much the commutator has worn down, here's a side picture of the remaining 3 bits of copper
The only way to try and prize a little more life out of it was to clean up the commutator - remove those 3 left over ridges and take a small amount off the remains of the commutator on a lathe. I found someone to do that, he only took off a small amount however you can see the shiny new copper (though it's now worn down so much that each copper piece is now two pieces - where there is some sort of locking thing in the middle of what was the old commutator contacts)
I have new bearings, the guy at the motor repair place said to stick them in an oven for 15 mins at 150C and then they just slide onto the shaft. Interesting point to note is that rewind prices are quite high because they have to wind the copper coils by hand - there's 12 in these motors.
Incidentally the commutator is knackered however the motor man did say that there is a company in London -
Robson & Francis - who not only rewind and repair motors but will also make and supply parts for them - including commutators. Their website is very specific that they will make parts in order for you to perform your own repairs. I've not managed to speak with them yet as their voicemail says the office is closed until the 10th but i will be making contact.
I'm hoping to get the motor back into the HPU in the next few days and will update this thread.