I've tried submitting several CT600 for companies with year ends 30 April 2011 and later. I use Forbes software to submit all my tax returns.
I believe that it is not possible to submit such CT600's using HMRC's own online submission portall as they have not yet got round to reprogramming it to accept the new rate of corporation tax.
David Forbes tells me that this is the reason why my submissions fail - and I have no reason to doubt him.
Moreover, it appears that we will have to wait until around December 2011 until HMRC get their act together before these CT600's can be submitted.
One thing I do find strange is that I hve not yet heard anyone else mention this problem.
Am I the only one? I'd be interested to know that I'm not alone.