Lawrence,
The suggestion of a local carpet store must be your best bet but if that doesn't work out you can buy 3.35sq m for £20.99 plus VAT (and ? postage?) from the first website below or 5.5sq m is available on the auction site for £54.99 - see below or a "sample" at £0.99 but you would need to find out how large the sample is and you might be able to ask for a larger than normal sample!.
https://www.buy-carpet-underlay.co.u...system_10.html
https://www.ebay.co.uk/itm/Duralay-S....c100033.m2042