This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

Oils Technical Manual

Pipelines: What is the difference between critical and non-critical interfaces? : How is the duty rate decided?

The interface is a mix and is separated and when stored in interface tanks (as opposed to adding into a compatible product), it is stored by flash-point.

‘Low flash’ point oil is volatile and will ignite at a low temperature - this oil can be taken as equivalent to light oil for duty purposes. Similarly, ‘High flash’ point oil is equivalent to heavy oil and duty is taken as being at that rate. However, some operators determine the category by density. (A density below 0.79 is taken as being light oil).

It is essential that interface oil keeps the same duty type declared on the Duty Adjustment Statement. Accordingly, if for example the interface is high flash and declared as heavy oil it cannot later be reclassified because its density is found to be below 0.79.