EU Subcontract Processing - Post Brexit HTCs

[Starfighter]

I have a UK based supplier making some parts for me. The parts require a specialist heat treatment done on them and the only approved facility to do this is in Germany. Post Brexit we have a problem with HTCs on the customs forms as the parts are shipped out process and returned to the UK. Both EU and UK customers are hitting the parts with import duties etc.

Does anyone know where we can get details of how to send part in to Europe for subcontract processing and limit duties to the value add of the heat treatment?