Fill rows downwards with column names

[Les]

Hi all,

I need to fill in coulmn A from row 1 downwards with column names e.g.

row1 A
row2 b etc, it works fine to drag it down, until you get AA ??
I need to go down until the last column name or unti desired.

Column B must be one ahead e.g.

A B
row29 AA AB
row30 AB AC

Any help would be much appreciated...