An answer from an expert -
http://news.bbc.co.uk/1/hi/business/8624932.stm
"There are no defined minimum standard requirements, such as there must be central heating or double glazed windows, but there is a housing health and safety rating system under Part 1 of the Housing Act 2004.
Under these regulations, houses can be defined as being a health hazard if there is excessive cold.
You could get the local authority housing standards team or the environmental health team to come and inspect the property when it is excessively cold. They can serve notices on the landlord to rectify the problem.
If the gas convectors and gas fire make the house warmer but are expensive to run, it might be the case that you consider moving to accommodation with better heating. If the heating is expensive but not classified as inadequate and not producing a health hazard, then there is an element of "buyer beware" for the tenant who chooses to take on a tenancy at such accommodation."
Doesn't sound too hopeful.