Generally Pub Car Parks seem a bit better than retail parks.
I dunno either to be honest. I would suggest the following:

1. The Lakeside - Bedford Road NN47YD
2. Queen Eleanor - Wootton NN4 0TG
3. Hungry Horse - Sixfields NN5 5QL

We also used to have TVR meets at

4. The Turnpike - Harpole NN7 4DD

All have big enough car parks, decent food if anybody wants to eat