you will find most of them offer a reasonable rate to new customers, so just go for the highest interest rate. next year you will probably have to look for another to transfer into though, as the rate if your existing one will plummet to virtually nothing.
cunts should be looking after existing customers, but they don't. the providers are being investigated by the FSA for that practice, bt in reality all it will end up doing is dropping all rates to the midpoint.