Not sure exactly how the out going msg should look like but try this. Also create a sitemap with your item Boiler_Light and if the mqtt msg is correct you should be able to turn the relay on and off from there.
Switch Boiler_Light "Light" <light> (Boiler, gLight) ["Lighting"] {mqtt=">[mosquitto:house/switch/37:command:*:1], <[mosquitto:house/switch/37:state:JSONPATH($.info.status)]"}