Who else should I visit?

Hint 1 of 17:

As always, visiting the scene of the crime, Mason's house, will do you good.

Hint 2 of 17:

You'll find out that Oswald's wife is staying with Dahlia Farmer.  See her.

Hint 3 of 17:

You will probably want to see anyone who was connected with the work that
Mason was doing for the Bank of England.

Hint 4 of 17:

Those places include The Treasury (not Mainquist's home) and the Bank of
England.

Hint 5 of 17:

Lionel Foxcroft also has some information.  But you'll really want to follow
up on the leads in Oswald's desk.