So, this is now sorted and she's officially a "daily" now. Was a matter of getting the system properly bled, there was air remaining in the system. Love the self bleeding system on the M3, this manual system sucks balls. But I'm there now, heater blows hot and the temp gauge doesn't budge from centre.
New synthetic oil and filter to match also done this weekend as well as a wash, she's looking as good as she's going to get I think (aka pretty average). Fuel and spark seem fine but I'll check them out over time. Idle is perfect and pull is what I'd expect for a slush-box.
So far $2,500 in and pretty happy.
Trans oil and filter next, Glenn's on my list for a call this week and if the price is right I'll get his guys to do that for me.