bugsy13:
If you definately want a 30 day sim only deal check out the network websites, but perhaps the most relevant issue is why are you seeking a different arrangement. Which specific improvemnets are you looking for over the current deals? How many calls/texts are the users making in a typical month and how much are they paying? Are they comfortable with a Direct Debit arrangment for the monthly charges.
If the limit is the most important feature than only PAYG offers that reliably. Only PAYG does not also offer the opportunity to scr*w a credit history for 6 years.
Up to you but please look at PAYG in conjunction with a sim free phone. Look at play.com to get an idea of the cost of a sim free phone. Also note that Asda Mobile network phones are sold unlocked.