iPhone 6S 16GB (refurbished) FREE upfront, 3G data, unlimited calls and texts - £28.50 per month at e2save
Upfront cost usually £50 but voucher makes it FREE
Voucher code: 6SREFURB50
Total cost of 24 month contract: £684
Cheaper than a brand new 6S by some distance as far as I can see. 3GB is a decent amount of data for the money too.