Had the car back into the local Ford Dealer, and after a lot od investigation it turns out it was indeed the ABS module that was at fault.
The module is split into 2 parts: a mechanical side and an electronic side. The fault on my car was with the mechanical side which houses the valves for the brake lines. Because of this no fault was being thrown up by the electronic side, and as a result no fault codes were stored and no lights were illuminated.
The constant clicking I could hear was the motor on the pump operating to build back up the pressure back up in the valve, which was leaking away.
The "block" as the call it came in at £182 + vat plus fitting - a lot cheaper than the whole module.
So, the issue is now resolved! Thanks to all those who replied!
Yes, as soon as the brakes are applied, even a little the abs kick in with the pedal pulsing and the pump operating.
I will recheck sensors again but the dealer said they were all ok on the IDS system.
I understand that my car, a Mk 2.5 uses a magnetic ring instead of a toothed ring. I did check the wheel bearings but they were smooth with no play, so thought that they couldn't be the issue. ..could they?