The Guardian: source for 35 page "dossier" is "consistently reliable, meticulous,

[Grey Lemercier]

33. "The Guardian has not been able to confirm the veracity of the documents contents"

The material, which has been seen by the Guardian, is a series of reports on Trump’s relationship with Moscow. They were drawn up by a former western counter-intelligence official, now working as a private consultant. BuzzFeed on Tuesday published the documents, which it said were “unverified and potentially unverifiable”.

The Guardian has not been able to confirm the veracity of the documents’ contents.......

Also, the OP title is tad misleading

the Guardian itself did not say the source for 35 page "dossier" is "consistently reliable, meticulous"

that was a quote the publish from an unnamed US source

An official in the US administration who spoke to the Guardian described the source who wrote the intelligence report as consistently reliable, meticulous and well-informed, with a reputation for having extensive Russian contacts.

An ex British intelligence agent would never so sloppily misname call the Alfa Bank the "Alpha Group"

also

the report mentions the World Cup soccer tournament

we never use soccer, it is football

and (as the Guardian did in in this very article) we do not add soccer or even football to the WC

it is simply a given that the World Cup is the football World Cup

just like Americans never call the Super Bowl the "American Football Super Bowl"

here is the Guardian:

It claims that Trump had declined “various sweetener real estate deals offered him in Russia” especially in developments linked to the 2018 World Cup finals but that....

Read the report, it is horrifically written, no MI6 or MI5 agent (or any other British intelligence person) would write in such a sloppy, error-ridden manner:

https://www.documentcloud.org/documents/3259984-Trump-Intelligence-Allegations.html