This was advertised on a flier that came with a trial subscription of LOVEFiLM. You'll receive 20% off British Gas HomeCare for the first year, which works out at £10.40 /month for HomeCare 100. In the "Quote Reference Number" box enter "discthree". It took about two weeks for the welcome pack to arrive and then you phone up to book your initial inspection / annual service.
HomeCare 100 @ £10.40
HomeCare 200 @ £13.60
HomeCare 300 @ £18.00
HomeCare 400 @ £21.60
Also while you're online, why not see if you can save a bit on those energy bills...http://www.whichswitch.co.uk/
(EDF Energy's Online Energy V5 worked out the cheapest for me)