After all my recent maundering about there being no low powered systems suitable for backup servers I did a bit of lateral thinking and looked at NAS servers again (I had looked a couple of years ago). They have got a lot cheaper now and there seem to be two or three which offer what I'm after (specifically Linux/NFS support).
Synaptic, Qnap and IcyBox all offer NAS servers which handle NFS as well as SMB shares. Synaptic and Qnap go beyond that and offer SSH access to the server, a web server, PHP/mySql and python even! It may be that the IcyBox servers do as well but I haven't investigated in detail.
Power consumption is pretty low, the Synaptic DS209 for example, with two 750Gb disks claims 25 watts when active, 10 watts when idle.
Prices are in the £100 to £200 range without disk drive, adding a big SATA disk will only be £70 or so.
I think there will be one replacing my garage linux box quite soon and I can use the garage system as my general purpose test bed.