"Vintagebug" and all others ! Your responses are very informative and greatly appreciated. I do belong to the Historic VW Club and am aware of their valuation offer but the current edition of the magazine gives no indication of the appropriate person(s) to contact. I live very close to Westside Motors in Woodford Green and although I've had no previous dealings with them I received a warm welcome when I called on them on Friday from Steve Moran (who himself is a long-term owner of a blue RHD '56 sunroof) with strong recommendations to avoid pitfalls he had experienced himself in getting suitable cover. However, even he as a long-established, authoritative VW specialist with specific knowledge of the 'Oval Era' was unable to suggest a value. I've e-mailed the Historic VWOC and await their directions about how to proceed and who to contact.
Just out to get some photos in the sunshine - Thanks again for the valued advice !