I'm not 100% sure but I know HMRC are having on-line down time at the beginning of April to move into the new tax year, so maybe from 6 April onwards you should be ok.
As soon as I have completed the last payroll I like to get the on line filing done as soon as possible in case of any problems, but HMRC never make this very easy.
I have a couple of clients who I use the HMRC payroll disc for and I dont think it will let you start the new year until you have filed this year.