Thanks Jeffery. I was searching the internet on how to “purge” a chimney! LOL
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I do… it was a spelling mistake… Thank you. However, I do wonder now, are you the same guy who thinks the minimum requirement for a fire separation in an attached garage is a minimum of Type X sheetrock on all the walls? and that an attic is a habitable space? I hope not 
Hey Jeffrey, hope you are doing great! was waiting for you guys to chime in and help the guy out more than I could.
A builder (not roofer) came to site today and said it is entirely
portland cement parging issue. Not a roof / chimney issue.
This builder said lime mortar in the pointing should not have been
slurry coated using portland cement. You also said this. It stops
the breathing. He wants the slurry has to be removed.
He helped by removing the blocked vents using a Kango power tool and looking at
the photo where the vents used to be. It took him only a few minutes.
He did not want to remove the portland cement from the wall using
Kango or other power tools. He told me if I wanted to remove the cement
then I should get (as in purchase) “core chisle” or “scambler”. He
did not know about removing cement using acids.
I am thankful the vents are there in the attic / loft.
Cannot find on the internet anything much about “core chisle” or “scambler”
though?
It shall be long job scraping the portland cement one brick at a time.
A few scraping tools would be needed and quite a few hours and days.
Then he shall consider lime mortar re-pointing of the bricks.
So he is okay with all the cracking on the chimney and its flashing details above the roof?
Why were the original vents closed?
The external chimney / roof requires some work from roofers’ view.
(Flashing is non-toxic and ecological and not lead flashing). Though
from builder’s opinion today on site the damp in attic / loft is
directly due to the slurry portland cement sealing / muzzling the lime
mortar brick pointing from breathing.
Builder’s story is that it’s a breathing issue (trapped moisture) not
rainwater ingress issue at above roof line. Not so much focusing on
condensation either. The builder said if it was a rainwater ingress
then you would see a damp stain on bricks also at the top but there is
only moisture in the mortar pointing lines.
The tool for removing the parging is this: Scutch Chisel
Interesting info and tool: https://www.amazon.co.uk/slp/scutch-chisel/dkyo55awprzy9rj 
Good luck with your task, MS.
Perhaps, this is what he meant by “scrambler”?
But, it seems that theparging would have to be somewhat loose or used to clean up the bricks after the scutch chisel was used.
Thanks jjonas, if I ever hear that term I will know what to expect. Like any other trade, the terms and jargon are Important.
Should I use portland cement to do re-pointing and parging / rendering
on the outside chimney stack or should I use lime? Normally on around
100 year old building lime was originally used.
Roofers and builders are usually into portland cement, some fear the
lime burns. Only a hard to find classical builder or roofer prefers
lime.
Also lime is not recommended for use during Winter and chemicals are
not recommended either. Portland cement can be used in Winter by
putting chemicals.