Had this problem last year, posted on here with no replies; now you all seem to have the bug (funny old world). Good news is that on my 1.6e (99V), I disconnected the sensor to the motor which stops the "clicking". This is really simple to do.
Remove the screws on the RHS panel, at rear of engine bay,(torx head). Disconnect the small connector (only 2 there, the other one is quite large), the motor won't try and engage. Spoke to a garage and told it is quite a common fault. £350-£700 to fix depending on parts and a quick fix rarely works for long. As Ivor said perviously, it is only needed when reversing. This will keep you sane till you have the cash to repair.
[/B][U] IF YOU ARE UNSURE ABOUT VEHICLE ELECTRICS, TAKE IT TO A PROFESSIONAL.
Just done this & now there is no knocking when i start the engine. For anyone else who does this the smaller connector is white that you have to pop out. It is underneath the panel is right next to the bas of the windscreen.