Formatting UK postcodes

[Bobjonson]

I have a column of 12,000 uk postcodes without the usual space inserted before the final three digits. e.g NG75HY instead of NG7 5HY and NG162GD instead of NG16 2GD.
Is there a formatting tool or macro I can use to insert the space in the correct place?

[NBVC]

If it is always before the 3rd last character, then assuming you are starting in A1, enter formula in B1:

=REPLACE(A1,LEN(A1)-2,0," ")

copied down.

Then, if desired, you can copy this column and paste special >> values over the original and delete the new column.