I'll give you a quick synopsis Gary to help you develop your own skills.
The key problem is finding out where the last space is, or how many spaces there are, and this is done by looking at the length of the cell value if it didn't have any spaces in it ( the LEN(SUBSTITUTE(A1," ","")) ...) and taking that figure away from the length of the whole cell ( LEN(A1)-LEN(SUBSTITUTE(A1," ","")) ...). So for John Doe we get 1, John A.Downer 2, John A. and Jane Smithhaven 4).
We can then use SUBSTITUTE again to replace a paricular instance of a string with another, in other words, replace the final space with a non-used character, I use CHAR(1). So SUBSTITUTE(A1," ",CHAR(1),LEN(A1)-LEN(SUBSTITUTE(A1," ",""))) changes just the last space.
We then use FIND to determine the location of that unused character, FIND(CHAR(1),SUBSTITUTE(A1," ",CHAR(1),LEN(A1)-LEN(SUBSTITUTE(A1," ","")))) so we know where to extract the data from.
And finally, we use MID to get that last space +1 as our start point, and I use 99 as the length because MID does not choke if the length is too long, so we get away with a largeish arbitrary number, MID(A1,FIND(CHAR(1),SUBSTITUTE(A1," ",CHAR(1),LEN(A1)-LEN(SUBSTITUTE(A1," ",""))))+1,99)
Hope that helps.