I got sent an email from giffgaff stating that they would send me a SIM (I didn't request this btw) to give to friends/family.
This was about a week and a half ago lol.
Here is the email:
As giffgaff is all about keeping costs down, we rely on our members
recommending giffgaff to their friends and family – that way we don't have to
spend lots of money getting new members on board through glossy advertising
We were a little bit cheeky and popped an extra SIM in the post for you
to pass on, just in case you had any friends or family who've just come off a
mobile contract or paying too much on ‘Pay as you go'.
Don't worry – this is just a one off, promise. Plus hopefully it'll help you get
£10 extra credit this month – by simply connecting your first friend before 31st