Hi Michael.

I loaded up the tank model to extract the objects but I thought I would try one more time and, wouldn't you know, they united successfully! So I don't know why the problem originally occurred or what changed to make the union successful.

BTW I am also working on another model and encountered a situation where two objects wouldn't unite using boolean even though they were clearly intersecting. Tried a few times with no luck then looked at the properties of the objects and realised one was just a joined surface, even though it looked like a solid( I had used trim lines to divide up a cylinder). I must remember to check that all objects are solids before using boolean in future!!

Thanks for your interest.

Cheers
Malcolm