Moved into a property a week ago and noticed the whole week the bath tap for hot water goes lukewarm, cold for few seconds then back to lukewarm. All other taps will go boiling hot. But the bath tap has been having this issue since we moved in the property. I've reset, turned on and off the boiler and still same issue. The thermostat on the boiler when hot water is on only goes to volume 1 out of 4. That's with all taps (boiling or lukewarm). How do i get the bath water tap to go boiling hot as i can't relax without it.