Mann published the hockey stick graph concerning the Northern Hempishere.

The Hockey stick graph is garbage. OR the more polite way of saying that is "the conclusions are unsupported by the data".

The hockey stick graph indicates a warmer LIA in the northern hemisphere than any reliable evidence shows. Therefore Mann fits the qualifications of your request:

"Show me a climatologist who denies the Northern hemisphere in many/most parts got colder in the LIA."

Your New Scientist article does not address this and it an article full mistakes:
"The "hockey stick" graph, which shows a rapid rise in world temperatures over recent decades"

This of course is wrong. The hockey attempts to show a centuries long record- rather than any information regarding "recent decades".
So first sentence is wrong, I shall continue?
"They cite an anomaly in the graph - it does not record a dip in temperature between 1200 and 1850 - as reason to ditch the whole thing."

As far as I know this is also wrong, though I have no idea who the "they" is. And obviously this fault is due to article not identifying who "they" are.

And the article you provide further evidence of:
"Michael Mann at Pennsylvania State University in University Park, who constructed the hockey stick graph, has always argued that if this were so, the little ice age would not show up on a global temperature record"

Nor would it show up in his graph of the Northern Hemisphere. Which once again, regarding it, "the conclusions are unsupported by the data" as my reference indicated.