OH and I just recently did a survey for the NHS data folks (NatCen Social Research) that was in two parts. First part was health and lifestyle questionnaire and height/weight measuring, second part was more questions and blood pressure and blood testing. We got paid the grand total of..... a £5 Argos voucher.
We were picked because after a flyer through the door we agreed to take part, all questions and tests were done in our own home so it was no big deal for us to do.