If the accounting period is greater than 12 months then more than one CT600 submission will be required, as each can only be used to submit for up to 12 months. Attach the iXBRL Accounts file only to the first submission, and select the ‘PoA differs from AP-a/cs with sep rtn’ option for all other returns.
Further guidance on how to deal with computations for long accounting periods can be found in the HMRC guidance document: