Fresh

Fresh, like most pay-as-you-go tariffs, is not for people that talk a lot on their phones. But it's perfect for people like me, who mainly use SMS and need a number to be reached at.
Their rate is a flat 15p a minute to every network, with text messages at 5p. From time to time they have special offers and they reduce the rate for all users - they charged half rates for about a month last spring, very nice of them!!
International text messages are also allowed (sorry, I can't remember the rate) but you need to pay a very large deposit (I think £100) if you want to make international calls.

The starting package costs £20, and it is all air time - you pay nothing for the SIM card itself. The service started working about 5 minutes after I put the card in my phone and so far I've had no problems with the network.

The only problem is going overdrawn. This is pretty easy to happen - to check your balance you need to call a 0800 number, that is not accessible from mobiles, you need a landline or telephone box.
For some reason Fresh allows you to make calls that will put your account under £0.00, but then they bar all outgoing calls and messages. Even if you top up your account, the bar is not lifted for 1-2 days.
Also: you can only buy your starting package with a credit/debit card. Even if you specifically say that you don't want your bills to be paid by direct debit, if you are overdrawn at the end of the month, that amount will be charged on the card that you bought the SIM with.

However, their customer service team is very helpful and polite and the telephone automated payment system is very easy to use. A warning: always top up your Fresh card over the phone. Carphone warehouse queues are notoriously slow moving!