Doesn't look that much of an issue to me. The tariffs are all here.
http://www.manxgas.com/pdfs-tariffs/0412%20tar ifwebdocs.pdf
You can download this.
Depending where (which town) you are on the island depends which sort of gas you get - this is explained at the top - A, B and C.
Then for each of them there are 3 tariffs depending what you use the gas for. Presumably this means (and I've never been to the IoM) that you all have a meters which 3 dials on them that feed 3 sets of pipes? - otherwise I haven't the foggiest how IoMG measure this.
Take the dial readings now, take them again again each day and get your 4 digit calculator to work out how much to pay.
The standing charge is about �18 per quarter in total to add to the above All the tariffs include the 5% VAT