DISCLAIMER: Make sure you have all your data backed up before proceeding with the following. Not that this will cause an issue, but computers are not too be trusted! Always backup before screwing around with anything
You can sort this by doing the following:
Preparation Step 0:
Make sure you have installed onto your PDA some Registry software (such as PHM Registry Edits)
http://www.phm.lu/Products/PocketPC/RegEdit/
Step 1.
Go to the following folder in the registry with the path: HKEY_Current_User\Software\Microsoft\MAPI\
Edit the key called "AttachPath".
Give the AttachPath the name of your current card
In my case "SD Card" and you can rename the paths to something sensible from it's inital ridiculous "IPAQ File Store\Inbox.mstxxx.xxxx\Mail Attachments"
I changed mine to \SD Card\Outlook\MailAttachments\
Step 2.
If you now sync any Outlook items with attachments they will be stored in your new location.
Problems
If you get the same issue as me, during a soft reset your PDA Outlook attempts to write to the SD Card location when it starts just to confirm that it's there. The problem is that the PDA hasn't yet detected the SD Card and so it creates the Folder \SD Card\Outlook\MailAttachments\ and then when it does pick up the card it names it \SD Card2\
Solution:
Remove the SD Card, delete the empty folder \SD Card\ and reinsert the SD Card.
It will detect it and label it with the original \SD Card\ name with no daft numbers.
Now when you sync your mail items they will be stored again on the SD Card.
That's 2 hours of my life wasted with that problem soving.
Thanks