How to account for movt of goods from UK to Spain, where goods are invoiced free of charge

We are a UK sheet manufacturer and we sell throughout Europe.  We have a situation whereby we are having some sheet made for us by a party in Spain.  In order to make the sheet, we are sending them some raw materials for them to use.  These raw materials will form part of the final product that they will send back to us.

We are sending the raw materials free of charge, and when we are invoiced for the final product, the invoice price will reflect that we have provided these raw materials to them.

Our issue is - how do we account for the fact that we are moving goods from the UK to Spain on our intrastat - given that the value we are invoicing for this is zero?

Has anyone else experienced something similar?