David,

Thanks for the tip. I have just read up the basics on this page -
http://www.excel-vba.com/excel-22-text-formulas.htm (a good read for
beginners)

I think I fully understand how to do it now!

One more thing -

I want to tell excel to do something DEPENDING on how long the number
is (I presume using the LEN formula)

so if the number is

1234, I want it to change to 1324 - I can do this using
=LEFT(A1,2)*100+RIGHT(A1)*10+MID(A1,3,1)

However, if the number is 12345, I want it to change to: 12354

(and if the LEN is neither 4 nor 5 digits long, I just want it to
leave the number alone)