Hi Adam, sounds like you're having a rough time of it. Can't answer all your questions, but with regards to the annual review, you will still get a gas/electric bill I presume, even after paying cash? Or are you in rented accomodation? The bill will prove how much more you are paying.
As for the petrol, I'm afraid it's just a matter of keeping your receipts for the next month so you can prove your outgoings, you still get a receipt for cash. At the worst, you would know roughly how much a litre fuel has gone up since you started your IVA, this should be able to tell you how much more you are paying a month.
I don't know if this has been much help, but you may have to start using your card more so you can prove the increases with your bank statements. Every company is different with their views on I & E, just out of interest, who are you with?
IVA completed April 2013