Problem solved. I ended up having to enlarge the holes to suit the 1/2" UNC bolts. I only found this out by chance by looking at the APV Safety Products website and coming across a work instruction. Still not 100% happy that I have to do this. I will speak to the guys at Hemco and see it they can reco my orginal units. I like the fact that Hemco are set up to test to the ADR standard.