Hot water and Central heating

It seems that the diverter valve is passing

Some boilers are plumbed in so that the Bathroom radiator is constantly heated by the overrun. When you are using the hot water the boiler may ignite to compensate for the use of the hot water - the radiator will then warm up as it is looped direct to the boiler. It's a good way of using the heat efficiently from the boiler - and it keeps the towels nice and warm. You may have a good system there. Of course it maybe that the radiators are plumbed into the hot water supply. If the radiator is cold and when you switch the hot tap on does the radiator heat up immediately and if you leave it running can you feel the radiator warming up quickly - check if the boiler is on. If it's a combi boiler then the boiler will come on as soon as you use the hot tap. If it's not it should take some time to fire up. If the rad heats up immediately - you may have a problem Houston !