I ordered a HP ProLiant MicroServer from Amazon and 4 x 2TB Samsung disks.
HP are offering £100 cashback if you get the server during this month, so including tax and delivery it’s costing me about £330 for 8 TB of raw storage (4p per GB — note that includes the server itself).
(Thanks Bryn M. Reeves for the tip off about these HP servers)
Update: Link to my old file server which will become the backup for the new one. The old server is still working well, but because it used an AMD Geode CPU it always was very slow.