I have no connection with these people, but have a look at
www.billmonitor.com
you can put in your usage details and phone you'd like and it will find you the most appropriate deals from over 7 million available!
you can put in your existing deals' online log in details and it will work out your usage over the last 3 months and suggest a contract based upon those details, it will; also update each month, so its handy if, like me, you are on a month to month contract and are contemplating tying into a contract either sim only or with a phone
it can also find the deals with free gifts such as playstations and tellys!