Hi Phil,
My Dann/Dans are definitely DANN originally from Cambridgeshire in the 1700s and then spread to Derby/Sheffield/Manchester by the end of the 1800s. My lot came from Manchester to Australia in early 1910s.
Hmm, very frustrating search for Dann/Dans/Dan in the UK - seems to be a very big dividing line of non-relationship between those Dann's down south in Sussex and those around Cambridge. It has been suggested that they were all originally called Dans - but it seems we may have to time-travel to check that fact!
Keep me in mind if you come across anything interesting,
All best,
Liz Dann