a quick check on your gas supply pressue depending on you having a gashob is. Put all your gas rings and cooker on full and note if the flames go down when you get the bioler to fire up.If they do you can assume your gas pressure is low, if they dont its ok. If you havent got a cooker ignore this,
look at the three port valve you will see a little lever push this across to the latch position this should give you heating and water and prove you have aproblem with this valve
your bioler obviously fires up to get your heating on. does it fire up when only hot wter is selected, i suspect not.if it doesnt i suspect your little silver box maked HW [hot water] it is actualy a motorised valve and is not sending asignal to the oiler and is stuck open
i am about to buy a new pump. i am happy with my heating. but would one of these digital pumps make any difference to my mister average gas bill.ps how do they work