The airport might be just off the A14, but LTNman's comment is a fair one. The terminal was hidden away and isn't that well signposted.
I have to agree. Here's the view from the main road, with the Marshall building on the right. There's a relatively small blue sign behind the traffic light.
You go into the car park, towards the back and turn left, going behind the white building. You then see this:
You go through this entrance which leads you even further back, round a corner and you finally see the terminal:
