Just thought I would let you all know how I've fixed this problem incase anyone has this difficulty in the future.
I purchased a ABS unit off Ebay. This had come off a 2000 Discovery 2. It looked identical and cost a fraction of the price of a new Defender one except there was one huge difference.. The ports were the brake lines run were in a different order.
My defender one: FL FR RL RR
Purchased Discovery: FL RR FR RL
Obviously this wouldn't work and would end up in a disaster if you had to use ABS or TC as the wrong wheels would lock up when the ECU wanted to brake a wheel in particular but sent the pressure off to a different wheel altogethor.
I took the punt and fitted it. When it was on I could possibly change the plumbing so that the correct lines went to the correct port on the modulator and all would be well. Turns out that it wasn't neccesary as the modulator still works the correct wheels when requested. Happy days.
Since I had my old Defender modulator now on the bench I though I would try to figure out what was wrong. I removed the black plastic cover to reveal the motor. Two allan bolts held the motor in place so I removed them and took the motor off. At the end of the driveshaft which pumps the two pistons there is a small ball bearing. This had completely crumbled with balls everywere. This is my problem and hopefully someone on here might be able to tell me were to find a new bearing for it?
Hope this may help someone out...
Tom Kirk