Welcome to the forum Digby. I'd start by searching here:- https://www.approvedworkshops.co.uk/
If you did decide to go the main dealer route you will have a "bit of a shock". Kool Classics https://www.koolclassics.com/page/eriba-caravan-services are your nearest, prices will be high and I'd expect they are booked up solid for many months. It is usual if you use a main dealer to book your following years service a year in advance. The majority of dealers will also not take on vans they have not supplied.

Colin