Just noticed this offer is back on Sky Rewards Piggy bank, basically cash in unused data in your piggy bank and get a voucher for Sky Store. 20gb gets you a free £10 voucher, or 10gb + £2 gets you the same. You could do the second option twice i.e. in 2 separate orders, gets you £20 credit for £4 spend and uses up 20GB of rolled data.
You need to login to your sky account, go to Piggybank rewards accessories section and you'll find the voucher option when you scroll down.
Note: It's a voucher they post to you, not a code.
Seems a good way to use up some rolled data though.