Actually, I had no problem Boolean union this in MoI it worked quite well, in some cases it is a matter of a picking in a sequence where in this case I would union only two pieces first then picking the next piece and union that and so on.

I guess it doesn't over load the calculations compared to picking the whole lot at once, I use this technique, not only in MoI, when things like this don't happen in one shot, I also use this technique when a string of edges won't fillet in one go.

Cheers