Hi there, when I reverse and brake sometimes the brakes seem to barley work! I can feel the ABS clicking on the pedal but the car hardly slows down even when brakin on tarmac with plenty of grip.

When the ABS doesnt kick in the car stops fine though and when going forwards the ABS functions propperly? Is this how the car is supposed to be of have i got a faulty ABS sensor or something??

Sounds like something is faulty to me. Can't think why the problem would only occur going backwards though. ABS sensors only detect wheel movement whether going forwards or backwards. What would worry me is if the problem effected the car when going forward, at speed in particular. If the ABS is cutting in when not needed that will seriously effect braking distances.
My advice: Get it checked ASAP.

By experience I know that a car in reverse has less braking power (effect). This is in my opinion normal as when braking the car's weight comes almost integrally on the front wheels having the discs, when reversing the car's weight goes more towards the back of the car where there are only drums and as these are ran in for forward motion it is possible the shoes dont push completely on the drums with less braking power as a result ? (Just thinking... don't shoot me if all this is bu..sh.. )

Willo, I agree that the car has less stopping power going backwards. However, the fault is an intermitent fault which worries me.