Despite working the weekend, I was able to flatten the welded repair on the boss (thanks everyone for all the helpful advice - and you were spot on Rich, the Ali does gum the file despite my best efforts: I used a sanding block in the end). Then I was able to get the head in my pillar drill for the guide hole, drilled it to size and then tapped with a M8 X 1.5 die, and put in the manifold stud.
View attachment 109198
Happy days: the inlet manifold sits on this perfectly!
So sit rep is that I am now at a point where I can re build the top of the engine! It's taken an awful long time, but I've learnt a tonne along the way, which was rather the whole point of the project!