Thank you all for your replies.
Apologies, in 1851 she was indeed with her daughter Keziah in Reading, not in Kingsey. Very likely that if she had said 'addenham in Kingsey it would have been recognised as Haddenham. But I was wondering if the 'Long' made it a different place.
I haven't seen any PRs for Haddenham, Buck8nghamshire at all, or any transcriptions, there don't appear to be any online. When I first looked, Familysearch came up with Haddenham, Cambridgeshire, which led me to posting on the Cambridgeshire board, not realising (or properly thinking!) that they were different places, hence the somewhat sarcastic comments on the Cambridgeshire board!! Fully justified, I might add.
If anybody could point the way to any, or do a look up for me, I would be extremely grateful. Sarah Clark was my 4G grandmother, I haven't got very far really with this family. If I had mtDNA testing, Sarah would be the earliest on that line that I have found so far.
Regards Margaret