Apparently they don't recommend slicing, something about a performance hit and increased wear on the discs. I've decided to leave it as the following...

Originally 14 x 500GB discs, 7 per controller

The new config:

2 x 500GB drives as a RAID 1 pair on controller 1 for metadata
6 x 500GB drives as a RAID 5 set on controller 2 as the user data volume

The RAID 5 set is 2.27TB which means I'm only wasting 270GB as the size limit on the LUN is 2TB

This leaves 1 disc from controller 2 and 5 discs from controller 1. I'm therefore leaving it at 2TB until the new financial year when I'll buy a 3.5TB XRaid (7 x 500GB drives on controller and nothing on the other) whip a disc out of it and put the 6 other discs on the other controller. That would result in 2 x 6 disc RAID 5 arrays which I can add to the SAN.

I should therefore have 6TB at my disposal plus a cold spare drive module.

It's not quite what I wanted, but it's probably the cheapest solution which will give me a working system now.