Hello Purplebus
When I first saw your pictures of the mystery caravan, the design of the front and rear windows overhanging the lower wall panels struck a chord with me, so I did some google picture searches. Unfortunately whilst I found a manufacture (Thomson) who definitely used the overflying window designs front and back, other key features did not match the information.
However Whilst looking up Thomson, I found a website which seems to carry a lot of information about vintage caravans, including some quite obscure makes.
What I can tell you is, becasue on the front left hand side of the caravan there is an NCC's Approved design sticker, it must have been a production line model of caravan for the UK market.
You might be lucky and find a data plate on the caravans A frame.
Sorry I cant be more helpful on this occasion.