errr as they are Dutch employees working from a Dutch base it is not allowed to have the work they are supposed to do (ie flying AMS based aircraft) with non based (foreign) crew. So UK crew doing the normal routes: no problem. UK crew doing "Dutch"routes... Interesting and food for lawyers. The Dutch law calls this the "onderkruipers verbod" which translates to "creepie crawlies prohibition" when (ab)using google translate.. which is close to:
