Manifolds include auto-air vents... which work, because they are always situated higher up than the UFH loops built into the screed.
However, when you start to use them with radiators, that might not be the case. So I'm curious, did you position it higher up the wall to try and keep the vent above the level of the radiators, or do you just bleed any air in the system using the radiators themselves (as you would with any standard sealed system). I guess the manifold vent probably just becomes redundant, unless you can fix it higher up the wall.
I'm about to undertake the same approach in my build, but am undecided on this particular point. I'll probably keep it low and just rely on occasional radiator bleeds.