I'm building a road bike and although I'm OK with MTB bits , I'm not familiar with roadie bits. Can anyone tell me what the difference is between type A and type B Shimano Dura Ace chain rings and why aren't they compatable.

It's to do with where the shifting "gates" in the chainrings are. If you mix and max A & B type rings then the "gates" don't line up properly and shifting is compromised.

Thanks for that Simon, I'll probably be back at a later date with more questions.

Simon is correct. A type is 42 inner and B type is 39 inner. The ramps on the outer ring change to accomodate the difference in jump between the rings. There is also E type which is 44 inner but that is not so common.