I have used e2save for many years now and have used the chequeback schemes they use. As long as you send a copy of the relevent bills in within a month of getting them they always pay up. I would not trust some of the newspaper advertised offers but e2save are a website of Carphone Warehouse, so is a company to trust (unless you want Broadband!). You also have the bonus of being able to take any faulty phones to a CPW store as I did a few years back....
i would reccomend them to anyone!