No hot water, heating fine.?

Check if you have one radiator (usually the bathroom) getting warm when hot water only is on, some system are piped with a loop through a rad from the hot water, if this is the case, the water is taking the easiest route to the rad. It is a water resistance problem somewhere in any event, this is just one suggestion. You may have inadvertently linked flow to flow on hot water, this will let the water flow to a degree but will shut the boiler down quickly when the pressure backs up and the boiler overheats, if the hot water is not returning but is getting to the cylinder, it is one of the above.