Ah, so good flow upstairs with proper tap, poor flow/no flow/spluttering downstairs? That may eliminate a blockage with the cylinder and CWS tank.
How long to fill a 1L container from this working tap? The flow may still not be as good as it could be.
Getting difficult to help now.
Even a high pressure tap downstairs should still give a steady, constant flow from a low pressure supply, even in 15mm - it just won't be very fast, and should certainly not stop & splutter due to low pressure alone.
The T from the top of the cylinder goes (1) down to your taps and (2) up and over the CWS tank. This is correct.
So your plumber put a stop-end over the vent, and everything worked OK, but spluttering started as soon as the stop-end was removed. If the stop-end is left on, the vent cannot empty and remains full of water.
This seems to indicate that the vent is the source of the air.
Your upstairs tap works, but downstairs taps splutter.
Since your downstairs taps are at a higher pressure (more head), I would expect the flow to be greater than upstairs. It could be that the flow to downstairs is sufficient to empty the vent of water, but the flow to upstairs does not.
Which points to either
(1) cylinder not refilling fast enough, which plumber appears to have checked & eliminated; or
(2) a restriction between the top of the cylinder and the T.
After this, I'm out of ideas.
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