I've come back to this little puzzle.
So far we have Richard Dean who died after 19 Nov 1836 and before 1 July 1837, who had a wife Mary, daughters Mary (b abt 1798, married John Derham), Elizabeth (b abt 1800, married Joseph Cooper) and Jane (b unknown date, no husband mentioned in her father's will) and son John.
I've found a death for a Mary Dean in Christchurch district in 1842 age 78 (b abt 1764) and another in 1844 age 70 (b abt 1774).
I have found the older Mary Dean in 1841, she is Independent, address New House off D?.... (can't read place) in Holdenhurst with one servant.
Piece: 398 Book:29 Folio: 24 Page Number: 14
Two possibilities for younger Mary
Piece: 397 Book: 3 Folio: 13 Page Number: 18 (this one is with a Tom Dean of same age so unlikely to be her.
Piece: 397 Book: 3 Folio 10 Page Number: 11 (possible, no occupation, record said widow, but has been crossed through, two younger females with different surnames in household ages 15 and 20 no occupations for them).
My feeling is that the older Mary is the one, which would widen up the birth year for the missing John and Jane and date of her marriage to Richard.
Can anyone find any possibilities for John and Jane in censuses please?