VIP rewards problem with discounted Sim data plan

[Gruffy3]

My VIP rewards tells me I can get a 10Gb SIM plan for £5/month. When I put it in the basket it comes up as £10. I'd actually like to buy 2 at this price. One for me and one for my wife. ( which is what we currently have but @nearly double the price for less Gb). We own our phones and are not locked into any contracts so this is not related to any contracts. 
Any ideas why the offer isn't honoured at checkout?

[caesarome]

@Gruffy3 

Are you a current mobile customer who has a Sky sim ?

[Gruffy3]

Yes. I have 2 SIMS. Both attached to my Sky account. 

[63johnw]

Hi @Gruffy3  do you realise if you order these then they are additional sims with new numbers on your account, those plans can't be added to your existing sims ?