You need a WRAS approved - Part L - filling loop - then .....
1. You need to shut off the mains water and change the valve that is connected to the mains water pipe - tee'ing into the 15mm pipe in your pic
2. depressurise the CH system and change the other valve that's connected to the CH system - tee'ing into the 22mm pipe
yes - you really need to find the leak - after a couple of years of constantly filling up the CH system with fresh water, it will have completely compromised the level of inhibitor in the system (if any to start with) and will be causing corrosion due to the fresh O2 being added all the time. If you bleed a radiator or that TF1 filter what colour is the system water?