Hi guys, just so you know. I decided to wipe the server completely again and start from scratch.
It felt like I was going round in circles and I'd probably start buggering something up if I kept on tinkering. So it's a case of fourth time lucky or bust.
Pleased to say that so far, so good. ISPConfig 3 is working and the server hasn't fallen over - yet (I'll see what happens with it as I start to put it through its paces).
More importantly, email is no longer the one sided conversation it used to be and thus is my problem here is essentially solved.
As for what the issue was before, I still don't know. And probably never will, I've done everything down to the letter as before and this time it works - so perhaps it was just a case of something not quite installing correctly last tme around.
Anyway, big thanks to you both.