I had an email last night from email@example.com asking me to fill in a survey that would take about 25 minutes on behalf of Cello Health. "The research will take the form of an online survey lasting approximately 25 mintutes. As a thank you for your time, an incentive of £5 will be offered on completion." (Mintutes is their error not mine!) I love filling in surveys - I'm odd like that - so I would have done it for free ... but I object to being TOLD there'll be a payment when in fact there wasn't one - they don't even ask for contact details so I can't see any way they could make a payment if they wanted to.
Just thought I'd let anyone else know in case they really wanted/needed that £5, and because it's just wrong to say you're offering an incentive if you don't! (And yes, I did get through the first section and was told I fitted the profile they were looking for and asked to fill in the whole thing - so I did complete the entire survey.)