just bought some of this underlay, its really hard to find on the internet but i found it on eBay;
http://cgi.ebay.co.uk/Axfelt-65-Super-Velvet-Carpet-Underlay-10m2-/180648082962?pt=UK_Home_Garden_FittedCarpets_Underlay_SM&hash=item2a0f76fe12
Just £3.65 per m2... but then you have your...