Who to assess: where regulations do not specify the person liable to pay the duty
In the few cases where regulations fix an excise duty point but do not specify the person liable to pay the duty at that duty point - see, for example, regulation 4(2)(f) of the Excise Goods (Holding, Movement, Warehousing and REDS) Regulations 1992 - you will need to choose the person to assess.
This should simply be a matter of deciding on someone with sufficient connection with the goods at the duty point. Where there is more than one person, you may need to make a decision based on who you consider most culpable.
As you may be called upon to explain why you chose to assess a particular person, you must be able to reasonably justify your choice.