I always think contract deals - whatever their price plans - are best for heavy users. I, too, am a light user and the best pay-as-you-go tariff available at the moment is ASDA Mobile. 4p per text, 8p per minute per call. That's it - 24/7, no catches. Just buy a SIM card from ASDA (49p), and it comes with an easy-to-read 'getting started' guide.
It's dead easy to port your number, too. I top up with �25 - lasts me a year!!!
The ASDA network piggy-backs on to the Vodaphone network, so coverage is universal.