Basically a salt lick isnt enough when on short over grazed grass like vast majority of uk horses - you need to add at least a tablespoon to their feed.
TT - I feed 2 tblspns daily but mums horse still licks metal stable fixtures. Has good gp supp plus extra zinc, copper, mag ox, salt and bicarbonate soda. He has access to salt & mineral blocks but never touches them.