Sealing cavity closer

[WWilts]

New build. Traditional block and brick/stone cavity wall.

Cavity closer as usual abuts inner & outer leafs. Windows/doors will project outwards beyond the cavity closer.

Builder will seal the cavity closer to the inner leaf.

Question:
Can the outer leaf / cavity closer junction be left without sealant? Perhaps relying on the window installers (not builder's subbies) to seal the window frame to the external leaf of masonry?

[SuperPav]

I never understood cavity closers - surely you just whack a strip of PIR in there with squirty foam round the edges, then it's completely sealed and you don't need to worry about whether the window installers will do a good job or not.  

 

But that probably says more about me than about the cavity closers!