If anyone is interested in the scheme I would advise them to Act quickly Its limited to 125000 places ( if I remember correctly) On a first come first served basis. More importantly as already mentioned don't use a big company you will get ripped off, even if they are offering even more money off a local installer should be able to beat their quote.
Also bear in mind if you do get the voucher and have the job done you will have to pay the installer the full amount. It's then upto you to claim back your £400 which will take about a month apparently.
If anyone wants any more advice your welcome to give me a ring my numbers on the website.