1.If Carlisle cannot repair the water ingress to your total satisfaction, then you have an agreement with them that you can reject the car, don`t you Colin?
2. The Engine Management light can be something simple like a need to replace an oxygen sensor, but whatever the problem the Dealer who sold you the car (Carlisle?) provided you with 12 months warranty, did they not?
Let events under 1. above unfold and under 2. likewise, as neither should involve you in spending money.
If events under 1. above lead you to rejecting the car, then the problem which has caused the problem under 2. will simply reinforce your decision to reject.
Please tell me if I have misunderstood your predicament Colin.