So i have been playing some more with FreeNas 8 under virtual box to make sure it is capable of doing everything i want before i comit to a lot of hardware, time and possible dataloss.
Today was the step of setting up shares and disk pools etc. All very straight forward bar one unknown detail. Dont think your clever by starting with really small disks. i tried 3 1Gb size virtual drives and then 3 2Gb drives both of which had issues when you try and add them to a pool. So i went for 3x 200Gb virtual drives and bobs your fathers brother.
So i added some data (1Gb iso file) via CIFS share which worked (if slowish).
Next i tried adding a forth disk, this time on a seperate controller (just to make things more difficult)
There are no obvious options for "adding in a disk" but after a 10 seccond google i found this helpful post and followed it.
All fine and dandy. Next stage will be to get this running on some actualy hardware and buy some big disks.