replacing a Harp HGC1 controller?

the heating system consist of a Baxi WM51 3RS boiler controlled by this Harp HGC1.
I am trying to replace this controller. The way this controller is wired up does not make sense. It would appear that it is wired up to control the gas valve of the boiler!!!!
Any suggestion????

Also where can I get the wiring diag for the Baxi WM52 3RS