I've just come off the phone with Sky asking them for a box with more storage than my 2 year old 500GB Sky+ HD box. After giving me their spiel about Sky Q for £199 and falling on deaf ears, the assistant was able to offer me a swap of my old box for a reconditioned 2TB model for £15 + £15 courier delivery, which I consider a very good deal.
FYI the assistant had to trick the system into thinking I already had a 2TB box, but he assured me that this was still a normal procedure rather than a dodge under the table deal - worth finding out for yourself.
Let me know how you get on in the comment section.
I have currently also no "invoice" to prove this, but I'll update with it, if or when I receive one.
Edit: member confirmed comes with 3 months warranty on the reconditioned hardware.
UPDATE: Initially marked as anecdotal, but now confirmed success by several members. And remember, if at first you don't succeed, brush yourself off and try again ;) - success seem to depend on the person you reach. Some of us had to call three/four times
UPDATE POST-DELIVERY: if you receive a Sky+ HD Box 2TB that's not wifi enabled call them again and request a wifi enabled box. They'll swap it over for you FOC.