Why am I am wide awake at this time of the morning....
Gas charges. If you've paid for the gas then I think they can't cut you off. Look at this link;
Gas Act 1995 (c. 45)
Look at the disconnection section ."Recovery of gas charges etc"
It talks about "for the supply of gas". Well, you've paid for the Gas but they want payment for something other than the gas supply you've had. Those extra charges are not for the supply of gas. I would write and point this out and say that any sworn statement to the relevant Magistates Court (needed before they get a warrant) will be challenged as perjury. Yeah, it's on oath from memory. You'll get a letter from the legal section in response and we can follow it up from there.
There's other legislation too;
Utilities Act 2000 (c. 27)
Read the bit about pre-payment meters - "A pre-payment meter installed by an authorised supplier on any premises shall not be used to recover any sum other than a sum owing to an authorised supplier in respect of the supply of gas to those premises or the provision of the meter.” It doesn't say "and any other charges!". So I think they don't have a leg to stand on. They can take you to County Court for the collection cost charges... oh yeah.