Just found as trying to decide whether to go for HTC Evo 3D or this (as now cheaper deal than Phone4U unless you are upgrading).
Price is £199.89 + £10 top-up
If you have already decided on the Arc S this seems a great PAYG deal.
Note: Only in white and with Vodaphone, O2 or T-Mobile top up.
Not sure on cashback so someone out there might know.