I have had the app for a while now (it's actually the second 1 I have had as I upgraded and lost all the credit in the app)

Anyway how do you spend your credit? I've got £2.53 and I bought san andreas the other night and it took the full amount of money out of my bank account rather than the subtracted amount

I've been using Google Opinion Rewards for a bit.
I'm not sure they allow partial purchases. I think the full amount would have to be covered by the Opinion Rewards credit. Surely there was a confirmation screen before the purchases went through?

I've built up a few quid and spent them the other day on android wear apps but yes, you can't use the credit as part of a bigger payment. If you have enough credit to cover a payment it shows up before suggesting a credit card to use.

ive not seen any surveys on this for months

It goes in fits and starts. I'll get nothing for a few weeks then several in a matter of days.