Right people I think I know the answer to this but let's see , is it normal to get the engine management light come on once you put a de-cat down pipe on.
And am I right in thinking that having a re-map should solve this ???????

 

Yes

 

If it's a 2001> car, then you're more than likely getting a P0420 fault code (Cat system efficiency below threshold) - This (or any other) code can be disabled when the car is remapped.

Ideally you should get the ecu remapped, but at the very least you should have the car checked with a WB for correct fuel mixture, and listened to with det cans.

STi models are prone to overboosting when the cat is removed, due to the reduction of exhaust backpressure.