It sounds like your hot water flow switch has stretched, taking more force to fire the boiler
Does this happen if you run the bath tap?
It does seem to take quite a bit longer for hot water to come through if I run any tap with the CH off demand, would it help if I changed the hot water flow switch and increased the water supply pressure via the stop tap
It does take longer becuase uour boiler has to start up and then get to temperature - usually taking 6 seconds or so before you get hot water to the tap. whereas when the heating is on then the boiler is lit and hot already
If it keep going hot and cold whilst ruinning the tap then get the flow switch changed