Cheers Ben.
I will seek to do that from now on. I think my only concern is getting the film to stick to the sponge - any tips on how to do this?
So far, when I have opened a cart I have found that the simple act of trying to get the old foam out results in them instantly crumbling and I've swept the lot into the bin
lesson learnt...
In the meantime I have just opened up a cart into which I placed the felt pads (which are sold as 8 track pads) and have added a strip of splice tape on top to see how it adheres. The cart itself is a slightly duff one which I am happy to sacrifice if it goes wrong.
Martin