I think he should take it further. This is serious and it should be investigated. I would like to know who sent the letter. I am extremely angry about it.
I agree - he should report it to the police .
This surely can not have been a Jeremy supporter as it would do a huge amount of harm.
I would say it is the opposite and to accuse Jeremy of getting someone to send it, in the press, without having proof is not a very professional thing to do either.
Whether or not they can do something with it or not is for them to decide - it is a threat and it has now involved other persons probably not even connected with that threat.
I am totally shocked to be honest.