King's £1.5m asking price has halted Hart and chairman Nigel Doughty's interest in the striker, whose contract runs out in 18 months time.
Collymore, however, is looking for a return to English football following a spell away from the game after 'retiring'. He notched 50 goals in 78 appearances before a £8.5m move to Liverpool in 1995.
It is believed Collymore is looking for a eight-month contract until the end of the season whereas clubs are more willing to offer a pay-as-you-play deal because of his past record.
Stan the Man has been linked to a number of First Division clubs and Southend Utd, another of his former clubs, are supposedly keen for a Colly return.
His agent Simon Kennedy described a fit Collymore as a 'considerable asset to any club' and told how the former Villa and Oviedo man is on an intensive training regime to regain his fitness.