What should be the Declared Value for Items coming into the UK from Outside the EU for repair

Question posted by Antony Stanbridge, on behalf of Livingston Ltd in TW11

I've aksed a similar question in relation to our rental business which was helpful although not conclusive)...we also have a calibration and repair business and a number of non-EU customers. These customers are sending their goods to us for repair and using a nominal value for the goods which doesnt seem to bear any relation to the repair cost of the open market value of the products themselves....(arguably the units are virtually worthless unless they work). But is there a guide or a frame work to calculate what the true declared value should be?
They obvouisly need to re-import back into their own countries and may not have a temporary export/import frame work that enables them to do so without high costs...but we want to remain within the bounds of UK law.