I just declined to review an EPSRC proposal that I was invited to review. I feel guilty about this because it's important to do this job reasonably frequently, in order to pay for the similar tasks I incur by submitting proposals.
I declined on grounds that my research expertise was not relevant. Which was the case, really. Of course, I perused the proposal to come to that conclusion. It looked like quite a strong proposal that would be stronger than most other ones in that research area. I've heard that the research area in question has already gotten lots of funding recently (can't verify that of course) which makes me think it questionable to support this proposal, although you can't have your recommendation be influenced on those grounds. Neither are you supposed to be influenced by the price of the research project (!!) for example the salary of the investigator(s) can be seen but should be ignored. Likewise whether they ask for one postdoc or three, the theory is that if they ask for three and they propose enough work, and propose to spend onough time supervising them, you should not mind the large number of competing proposals that will be denied funding of that one gets funded. Of course, reviewers are only human and these factors influence our decision.