On the ‘Company Details’ page there is a ‘Next Year End Date’ to be filled in. The pop up help for it says to use the Accounting Reference Date from companies house. When I look on companies house I have
Next Accounts Due: 15/04/2016
Next Return Due: 12/08/2015
You should see another field on Companies House, the “Accounting Reference Date”, e.g. 31/03. This is the day and month your accounting year will end. So in the case of 31/03, the next year end date would be 31/03/2016.