New Blackberry Priv for the best on-contract price I've seen so far.
3GB of 4G data, unlimited minutes/calls, O2 perks and free delivery.
If you're not a fan of the phone then fair enough, but I've been dying for a decent android phone with a physical keyboard, and this is it!
If you can find it cheaper, with the same or more data then let me know, but this equates to £10 per month for the contract if you deduct the phone price.
Updated Code: TENOFF10 gives £10 off the up-front price, bringing it down to £40.
£10 quidco is also possible, taking it down to £774 or less than £9 per month for the contract
*** Update ***
Spoke to O2, who confirmed the contracted data allowance can be increased mid-contract without much hassle and fairly cheaply. Upgrading from 3GB to 5GB would add "less than £5" to the monthly bill.
(£3 per month according to this thread: http://www.hotukdeals.com/deals/iphone-6s-64gb-o2-4g-100-off-using-code-blacknov15-unlimited-calls-texts-3gb-data-2319260)