Tom,
I had very similar symptoms to what you describe, (except no sound of
air leak) but the cause was the petrol  was bad (30% diesel content) in
our passat 1.8gt (digifant 2 fi)
Start up was okay repeatedly , but poor idle and no power, some hours
later followed by no start up, at all.
The plugs did appear to give a good spark. On a friends reccomendation
I first changed the plugs, there came a little improvement, but back to
square one - no power.
I drained about 5 litres of fuel and tried it out in a friends banger
and it spluttered even worse, so I was pretty sure the fuel was bad. The
friend was not much better!
Then I drained all the fuel and put in clean fuel, still no big
difference. Then I replaced the plugs for a second time and then
everything was back to normal.

I had tried to dry  and clean the the first set of new plugs  but they
still did not produce enough spark even with the clean fuel, I was
surprised by their sensitivity.
Replaced filters and oil later.

Maybe this may help, I can understand your frustrations after doing all
that work rebuilding an engine and it wont start. A similar thing
happened to me but I had done one mistake so stupid that I can not bring
myself to mention what.

good luck, Martin Kennelly.