Hello Niel,
A yellow sooty flame tells a gas engineer that there gas to air mixture is too rich.
It could be as Hamer suggest a regulator problem, but if all your other appliances are working correctly, that tends to discount that theory.
More likely is the primary air venturi tube has become blocked and needs a service.
The fact that you did not know that, means you need to have the appliance serviced by a gas competent person.