Surely the answer is yes... but I've just had a plumber adding in and moving
radiators etc. Near the end of the job I asked him casually 'so when does
the inhibitor go in'? meaning of course, 'don't take me for a numpty, I
know you haven't put any in yet, so don't think you can get away without'
and he says 'oh no, you don't need that these days; you had to use it with
the old vented systems but not with these sealed combi jobs. And anyway
there's no header tank to pour it into, to get it in the system'

As so often when I'm faced with an apparently experienced tradesman telling
me that black is actually white, I just caved and left it at that (despite
having personally injected inhibitor into a sealed system before using the
special syringe provided with the stuff, so of course it's 'possible').
This guy has been a qualified plumber all his life (now in his sixties) and
I'd feel very uncomfortable telling him he's talking b*ll*cks. But apart
from the syringe thing, is he? I suppose if no air can get in to the water,
corrosion is less likely isn't it? I'd planned on just injecting some
inhibitor myself later without letting on, for a quiet life, but do I need
to?

David