A3=16/05/2005
A4=A3+29

I did a fill down, past A30. So all down column A I have 29 being added
to the date in the cell above.

I replaced the formula in A28 with =Year(A27) But it just gave a date.
Then, I put the same foruma i.e. =Year(A27), into a cell within
column B, like B9 and I got the right answer.

Has filling down somehow tainted cells I filled in column A?!
Both A28 and B9 have the same formula, but only B9 gives the correct
result. A28 givesa date instead of a year, even though the forumla in
there is =Year(A27).