Thank you for this. I will look at it again later this afternoon. I have just re-measured the thickness of the spacer I used (actually a square nut) and it's more 3.72mm.
I've just measured the original spacer that I printed and it appears to be 16.7mm thick (measured with calipers) - not the 17.2 mm you quote above. It may be that my printer is not extruding enough to give the correct layer thicknesses. I'm new to this 3D printing lark so will need to check my extrusion rate.
I'll let you know how I get on - it might be a day or two though. Thanks again.
Edit @ 16:28: I've decided to return the dampers I have and buy a different set that are also listed as 6 mm and hope that is what will turn up.