Resident Evil 0

*sigh

I hoped this would be easier, but it's not as simple as unlocking Wesker mode (by completing the game).

You have to:
1) Complete the main game to unlock Wesker mode
2) Complete Wesker mode to unlock the costume.
3) Load any save game file (but oddly not a Wesker mode save file)
4) Open suitcase in your inventory (as Rebecca), equip and and POP.

The costume is Rebecca's from the Wesker mode game and seeing as you cannot switch costume in Wesker mode you obviously need to do it in a standard save file.

Something I learnt the hard way, please note when you beat Wesker Mode DO NOT overwrite your main game save, this will lock out your ability to replay the game normally, and lock you out of your other achievements progress.
Save in a slot that isn't your main cleared game, the Team Wesker outfit is an account wide unlock.