I would probably try and venture out for lunch at a pub somewhere, via minicab from the North Terminal.

Somewhere like 'The Cat Inn' would work well:

http://www.catinn.co.uk
http://www.telegraph.co.uk/travel/ho...el-review.html

which seems to have decent reviews and is about 12 miles from the airport

http://goo.gl/maps/Qlc07

If you have 5 hours should be plenty of time to get there and back and for a long lunch.