There is no 'p' in Vimto!
It is surprising how many people pronounce it as though it does. Vimpto.
If you were unfamiliar with this particular brand of cordial and you had only heard it pronounced with a 'p', how would you think it was spelled?
According to Paul Lawson's diary, he showed the bust of the Somerton man to the former nurse "Mrs Thompson" on July 26th, in his office. At this point in time Mr Lawson had only spoken to Jestyn on the telephone and she was using Prosper's surname. I think it entirely possible 'Thompson' was a simple spelling mistake.
This raises another question "Could Carl Thompsen be related to Prosper Thomson?" Perhaps his name was Carl Thomson, as has been suggested elsewhere?