New offer has popped up for Amex users, expires on 31/12/2014. In-store purchases only, includes gift cards.
This requires you to purchase a £50 PSN Card before purchasing the PlayStation Plus 12 Month Membership (but it still works out way cheaper than any of the other deals for this at the moment! £25!)
1. Load Argos Amex offer to your card (see loading instructions below).
2. Buy £50 PSN Card (you will get £25 statement credit back from Amex).
3. Purchase PlayStation Plus in the PlayStation Store on your PS3/PS4 (£39.99).
4. You now have 12 Months PlayStation Plus AND £10.01 PSN Credit left over, which costs you only £25!
How to load Amex offer onto your card:
Use Foursquare to load it to your card if it doesn't appear on your account.
If you haven't used Foursquare with your Amex card before.... (thanks to morgie)
1. Sign up for a Foursquare account at https://foursquare.com/
2. Sync your Amex and Foursquare accounts - https://sync.americanexpress.co.uk/
3. Install the Foursquare app on your smartphone.
4. Search for the merchant (Argos in this case) in the app. Click on the American Express offer and select 'Add to card'...the text on the button should then change to 'Added'.
5. Spend the required amount on your card in store.
6. Check in on your statement online a few days later to check for the spend and credit.
OP (thanks EmS138) with Amex offer: http://www.hotukdeals.com/deals/amex-offer-25-statement-credit-a-50-spend-argos-2086134