The process is quite complicated, but should be easy after doing it a couple of times.
Join Complete Savings. Pay £15 per month for membership. (You get a refund later when you meet some conditions.)
Then from Complete Savings buy egift cards for Argos at 20% off. Wait a few minutes for your ecode emails to arrive.
Then go to Argos online via Complete Savings cashback option, sign into your Argos account, for 10% cashback on your order from Argos, for sms notifications of your orders and FT numbers, and for extending your reservations if you need to.
Choose your items at Argos. Choose to pay now at Argos online. (If their are vouchers for your purchases these will be online codes sent to you via email from Argos).
Then buy your Argos Fast Track orders using the giftcard code. For items over £100, £50 or £25 orders this will give you 20% off your order, then 10% CB off whatever there is left to pay. So for a £120 item you pay £80 for your £100 gift card, then at a future date get 10% cashback from the £20 remaining payment. If your credit card also gives you cashback you will get more cashback from them.
Complete Savings cashback payouts are also a lot, lot, faster than most other CB sites.
Pick up your items from Argos Fast Track. Then send your e-receipt to Complete Savings to get your monthly fee refunded as a monthly cashback bonus.
Happy shopping. I have saved hundreds of pounds over the last year.