Hi and welcome daddy_boglin - congratulations on your Dandy.
One upholsterer I'm aware of is The Caravan & Boat Seat Cover Centre in Bristol - [You must be registered and logged in to see this link.]
I know folk who have use them and who would recommend them highly. I did get a quote once and it was quite expensive though.
The most cost effective solution might be to find a local person who has the necessary skills and equipment. Have you got a Streetlife network in your area? - [You must be registered and logged in to see this link.]
It's a good place to ask for this kind of thing.