The material that it needs to be must have plenty of 'texture', I have confirmed that simple acoustic foam is not enough. I think in my case the reason is that the TV is only about 15 cms from the wall, so foam is still allowing the sound to reflect a little. The material I first used in my original makeshift sloution was fabric from a thick sports wind cheater. The texture was like balls or tufts of fuzzy wool, except the fibre is a poly.
There is another possibility I am going to trial, is similar to this:
http://www.ikea.com/PIAimages/24097_PE094777_S3.jpg
It is a rug, but I'm now thinking of getting one (I can take one home on trial) and seeing if it works. If it does then I will 'stretch' the rug just like a canvas on a wooden frame and simply hang it behind the TV. I will try and source the rug type material in a fabric shop first though, as buying one ready made would be not cost effective.