no heating on sime format 80?

my sime boiler works fine for hot water but not for c/h. At first only c/h worked but since it was sorted so hot water would work, c/h wont work!
the micro switch that controls the flow for c/h has been swapped but still no go.
The bottom of boiler has been stripped and cleaned as has the heat exchange and also a new diaphragm fitted. any advice would be great, thanks