This seems like dumb question but I just want to make sure that my thinking is correct, but is the talent Weapon Training (Heavy) a single talent or do I have to purchase it for each weapon specialisation I'm trained in, in order to use that specialisation's heavy group?

It's a single Talent in OW.

To properly use a lascanon, you'd need Weapon Training (Las) and Weapon Training (Heavy) - but not Weapon Training (Heavy-Las).

That's what I thought and just wanted to make sure, thanks.