In
http://www.multimat.uk.com/store/products/underlay-xl reply to stu84:
Why not try a diy version of this. Just go to a foam retailer and get it made to measure.
The same kind of place where folk equip campervans.
Alternatively I have a 25 year old voyager, a bit leaky (fly) but would give you spare poles etc.
You could cut the old groundsheet out and seal it into your groundsheet (internally with seam seal).
Pm me if interested.