Companies carrying on “oil-related activities” are subject to both corporation tax and a “supplementary charge”. “Oil-related activities” are treated as a separate trade and the income from them represents “ring fence profits” on which corporation tax is charged. The “supplementary charge” is levied on “adjusted” ring fence profits, in calculating which financing costs are left out of account. Between 2006 and 2011, the supplementary charge amounted to 20% of adjusted ring fence profits. On 23 March 2011, however, it was announced that the supplementary charge would be increased to 32% from midnight. The change in rate was subsequently carried into effect by section 7 of the Finance Act 2011, which received the royal assent on 19 July 2011. Total E&P, previously Maersk Oil North Sea UK Limited and Maersk Oil UK Limited, carried on “oil-related activities” and so were subject to the supplementary charge. The question raised by the appeal is how much of each company’s adjusted ring fence profits for 2011 are liable to the charge at 20% and how much at 32%. The accounting period which ran from 1 January to 31 December 2011 and so straddled the point at which the supplementary charge was raised. The approach elected by Maersk Oil North Sea UK Limited and Maersk Oil UK Limited – an “actual” basis in place of the time apportionment basis – resulted in all the adjusted ring fence profits for the 2011 accounting period being allocated to the period before 24 March (“the Earlier Period”) rather than that from 24 March (“the Later Period”) and so in escaping the “new” 32% rate of supplementary charge. HMRC did not consider the basis on which Maersk Oil North Sea UK Limited and Maersk Oil UK Limited had approached apportionment of their adjusted ring fence profits to be “just and reasonable”. The Court of Appeal concluded that treating each time period as if they were two separate accounting periods, and allocating income, expenditure and allowances to the periods accordingly was just and reasonable. Capital allowances could be treated similarly for notionally separate periods ...