Go through the quidco link to get the £17.50 tariff, £49.99 for the phone + £80 quidco
If quidco tracks this works out at £16.25 a month which seems not a bad deal if you cant be bothered messing around with redemption offers.
Dolphin 25 - 24 months
monthly cost: £17.50
phone cost: £49.99
less £80 quidco