Officially, if you have an existing Orange PAYG sim for at least 3 months (or know someone who has), then give them the details and you'll get the SF for £78.99.
Depending on who you get, they'll either ask you to get the person who has the existing sim to ring them, only ask you for the phone number of the PAYG sim, or just take your word for it and not do any checks at all !
Worth ringing back if you dont get the price you want on the first go.
Yep, judging by many discussions it seems it depends which salesperson you get. Some might not offer a discount at all, whilst some seem to give it easily. I think I'm going to give them a try soon. (not on orange currently)