I have a client who also lives on a narrow boat, and have noticed large increases in costs in his case. I am also unfortunate to live out in the country where there is no piped gas, and have seen my LPG charges almost double over the last 2 years.
The discretion for the IP to reduce payments by up to 15% has only recently been introduced for new IVAs so as yours has been going for some time the IP will probably need to seek a formal variation of the IVA by the resolution of creditors at a newly convened meeting if you wish to seek a reduction in payments.
I did not, however, pick up from your post that you were actually looking for a reduction in payments, just that your IP was being enthusiastic about you producing receipts for expenditure. I don't think that his request is necessarily unreasonable, and you should be able to get receipts from your landlord if you are paying him fees. Why not write a letter confirming the amounts that you pay, and get your landlord to sign it?
Regards, Melanie Giles, Insolvency Practitioner