Been along time since there was a decent handset with decent amount of minutes for a decent cashback.
700 mins should suit most people and 200 txt also. Most clearance deals lately were 200 txts or only 200 mins.
Phones4u are considered reliable for cashback.
Vodafone sends their handsets unlocked - although this may come from phones4u stock which would be unlocked in any case.
Hope this helps someone.
Its worth reading Duckmagics website, an excellent guide to cashbacks. http://www.duckythemobilehunter.com/
Also use Quidco for an extra £50, which brings the contract to basically free with £10 profit.