Excel formula/function question

I've been asked to build a function to produce unique client identifiers:

The information needed is first and third letters of surname; first and fifth letters of first name, gender and date of birth. However if the surname is less than 3 letters or the forename less than 5, a "2" needs to be inserted, and if the  date of birth is 04/03/1968, the first zero need to be in the identifier.

This is the formula I have got so far which works fine but does not replace the blanks:

=LEFT(A1,1)& MID(A1,3,1)& LEFT(B1,1)& MID(B1,5,1)  &C1&D1

Thanks