OK, I have managed to find a workaround - I'll post it here for the benefit of other poor souls who buy the thing and have the same problem...
Get your contacts in several different contacts folders in outlook. eg:
Contacts - Personal
Contacts - Colleagues
Contacts - Work
Contacts - Useful
Also have a separate Contacts folder just for syncing with the P900. Start with it empty, and sync with P900 (Full sync settings - merge items). This will blank the phone's contacts (doesn't really seem to be merging to me, more copying PC to phone, but we'll ignore that...)
Now, COPY (not move) the first folder (eg. Contacts - Personal) into the Contacts folder to sync with the P900. Sync.
Go to the P900, open the contacts manager. Select "Unfiled", and "Select All", then Move all the contacts to the "Personal" Category.
Back to Outlook.
Copy the second folder (eg. Contacts - Colleagues)into the Contacts folder to syncwith the P900. DO NOT ERASE THE CONTACTS THAT YOU PREVIOUSLY PUT IN HERE or else they will be erased from the phone when you sync! Sync.
As before, go to the P900, open the contacts manager. Select "Unfiled", and "Select All", then Move all the contacts to the "Colleagues" Category.
Continue doing this until you have synced all the folders.
Pros:
Saves you going through each contact separately and selecting a category
Cons:
When you synchronise, your contacts will be sync'ed to the main Contacts folder in Outlook, not your subfolders, so if you ever need to sync them from scratch you will lose the categories, unless you keep your subfolders updated separately (which would be a real hassle 🙄 )
Hope that's of some help, seems to have sorted my problems to a certain extent.
Point of interest - doing a full backup includes contacts and folders, so that's one way to approach it. Is there any way of just restoring one or two files? I used to be able to do it on my Psion (also Symbian), and actually navigate around the file structure of the Psion from the PC, doing what I wanted with individual files. If I could just copy off the contacts data file and keep that backed up I would have more problems. It must be possible as it's the same OS, but what software do I need?
One other problem - the phone numbers that came with my phone from Orange don't seem to sync (they lose their numbers!), and now they have disappeared altogether.
1) Why!?
2) How do I get them back?