I've just finished cleaning the upholstery in our Puck L. We were thinking about getting it professionally cleaned but with further thought and some very helpful info on the internet I decided to give it a go myself. Having removed the upholstery from the van the trick, it seems, is to remove as much dirt and dust by vacuuming and thwacking the seat bottoms and backs with a fly swotter or similar (very enjoyable) before spot cleaning specific marks with a baby wipe. The next step is to gently wipe the upholstery with a microfibre cloth that has been semi wrung-out after soaking in a weak solution of warm water and wool/fabric washing liquid. Its been a warm sunny day so it didn't take long for the seat bottoms and backs to dry out and I'm very pleased with the result - just over 2 hours in total. So, cup of Tea or a Pint? no contest really, a pint and maybe one to follow - they're going to slip down rather well me thinks.
Just a note of caution, before embarking on a full clean test a small area of upholstery with the baby wipes and cleaning solution you intend to use just incase it reacts badly with the fabric or dye.