How to automatically assign next available number

Hi all,

I've worked my way through the best part of a new tendering register spreadsheet with the help of searching through this forum, though I'm at a bit of a loss with the last function I wish to add. Any help would be appreciated.

I am trying to assign an individual tendering number automatically from either a macro or a picklist if it can be done.

Column A will consist of 4ea x different business units i.e. Projects, URD, Communications & Workshop.

I would like to be able to assign an individual number for each division i.e.
A2:PROJECTS B2:P00001
A3:URD B3:U00001
A4:COMMUNICATIONS B4:C00001
A5:WORKSHOP B5:W00001
A6:PROJECTS B6:P00002
(and so on)

Once the a specific number has been used it can not be used again.

Any advice would be appreciated.