glo-worm 30ci, warm-cold shower?

What happens is the temperature of the shower fluctuates from warm to cold even when the shower dial is on maximum. after this we put the temperature of the hot water dial on the actual bolier, up past 'eco' into the red area. The temperature of the shower is still warm to cold, again even when we put the shower dial right to maximum. The kitchen tap produces hot water but it fluctuates from hot to warm, same with the bathroom taps. Pressure is 1.6 bar, and the light is yellow. have tried switching it off and then on, but did not make a difference. when the shower is on the bar does not change. please help ...........as cold showers are hard to handle. thanks