I saw this advertised about 2 months ago, i have now had it fitted and it was all free.
You do need to be on a quailfiying benefit, Most are listed for you to check first.but the company will tell you once you complete the online appication form.
its to do with the british gas promotion about making your house warmer, therefore reducing heating bills, My house is warmer.
Remember, your house needs a cavity for them to do the walls.
worth a try :)http://www.surveyukltd.co.uk/homeinsulation/index.html