HMRC notice 252 para 3.15 d(ii) states:
"You must add the following to the price you pay (unless they are already included):
(d) Goods and services provided free of charge or at reduced cost by the buyer. If you provide, directly or indirectly, any of the following, you must include in the customs value any part of the cost or value not included in the price charged to you by the seller:
(ii) tools, dies, moulds and similar items used in producing the imported goods, for example, tooling charges - there are various ways of apportioning these charges"
Does anyone know where HMRC gives any further information about the "various ways of apportioning these charges"?