Taking the lower cowling off to replace the SCAT hoses revealed that the biggest assortment of metric and UNF hardware you could imagine was being used to fasten the cowling. It appears to be pretty commonplace that metric hardware will gradually become diluted over time. I replaced some #10 UNF screws with their M5 metric equivalant, they are a little wider (.2mm) and now the correct length plus LAS were selling them at 8p (now 45p) which was a bit of a bargain!
Adventures with a Bölkow 207