auto-incrementing numbers with text prefixes in Access 2007

Hi,

Firstly let me appologise if this has been asked before (I've tried searching and not yet found anything I can use :()

My boss has asked me to automatically generate a number for leads that get entered into our DB via a form.

The problem is that these numbers need to be prefixed with either an AG or BG (depending on which of our 2 offices the lead relates to). Selecting A or B is easy enough but what I need help with is some code to find the highest number with the selected prefix and give the new record that number + 1.

For example you may have a table with these lead numbers:
AG1
AG2
BG1
BG2
BG3
AG3
AG4
AG5
AG6
AG7
BG4

If I were to add a new record relating to the AG office I want the code to see that AG7 is the current highest number and call my new record AG8.
Once that number has been generated I want it (including the prefix) stored in a field for that record (if possible I'd like this to be the primary key as all values will be unique).

I hope that makes sense, please ask if you need any more info.

Many thanks,
Gav