OT12450 | PRT: supplement - apportionment of expenditure
From HM Revenue & Customs · Oil Taxation Manual
OTA75\S3(6)
Where expenditure is incurred partly for one or more of the qualifying purposes set out in OTA75\S3(5) and partly for some non-qualifying purpose, an apportionment should be made in a just and reasonable manner, OTA75\S3(6). This is the provision which also governs apportionment of expenditure which is partly allowable under OTA75\S3(1) for a field and partly not, see OT09375.