Hi Eamon,
Sounds like a faulty sensor to me. You have 2 options:
Either scan with VAGCOM and it will tell you which sensor is faulty
or
With the ignition on (but no engine), engage reverse and jump out of the car after the constant beep. Put your ear close to each sensor and you should hear a clicking. The one that doesn't click is faulty.
Once you replace the faulty sensor that should cure the fault.