This was posted last year but is well worth a re-post as that post is 7 months old and the offer is still working.
Ordered some a couple of weeks ago and turned up today, although they say upto 28 days for delivery.
USE CODE EF3 and complete a few simple questions,
Where it asks for benefit reference number just enter your Ni number, and no you dont need to be a british gas customer(i'm not)
For NI number (reference thing) just enter N being number L being letter NN NN NN NN L and numbers and letter and it'll work