Test Pilots

Some of you might know that I am in a group working on an airworthy replica of the Heinkel He 51 biplane here in Germany. Though we are still lightyears away from the test-flights I have been approached by several (even well known) test pilots who would like to do flight-testing for us. I haven’t asked them this yet but would we be charged by the pilot for carrying these flights out or would the priviledge (and joy) of flying this unique aircraft be counted as payment in itself? What is it done like in the UK?

Regards
Peter