I eventually got sorted with the parts I needed. I got some little crimp pins, three pin mini housings and a new fan.
I fixed the crimp pins to the fan wires and tested it by touching the pins to a 12v battery from a remote control. The fan worked ok.
Next I put the pins into the plastic housing and connected it to the socket on the pc.
I then powered up the pc to see if it would work. I left the pc on for about 20 minutes but the fan didnt run.
Strangely when I spun the fan with my finger it started to run continuously until I stopped it with my finger. I then did nothing untill I spun it again.
There must be some sort of switching problem?