Ah! Well I didn't know I wanted it yet So far, £8 of underlay from the local carpet shop, and ten minutes of cutting and fitting. I believe in prototyping before going straight in at the deep end, so for that outlay (underlay?) I can drive about for a bit and give it a good listen. Seems ok so far:
'scuse the mess. I made a bit of a hash of the plastics in my boot with my mountain bike.