I have recently visited their website (www.diy.com) & they have a offer in conjunction with British Gas, where they will fill your cavity walls for only £198. I 'm unable to get it free as not on benefits & this therefore seemed a really good deal. I have been told approx 6 weeks from order to fit. Also INSTORE so I believe that you can get Diamond discount on it as well (minus 10%)
Also quidco if ordered online.