Leaking radiator valves in sealed system

[AndyBrown]

My combi-powered central heating system is about 2 years old and 2 radiator valves have started leaking within a week of each other.
Looking through this forum and others, I feel confident of replacing the 2 valves (or dismantling them and resealing with PTFE tape). I am not sure how to refill the rust inhibitor after the repair is done. I heard that there is a gel type of inhibitor, but when/where do I put it in. Also I am puzzled as to why these 2 valves started leaking almost at the same time (one is in the bathroom and the other the downstairs hallway)

 

[weareleeds]

Does it say bs2767 on plastic top of valve? if not change them all as the rest will start leaking as well

 

[easyflow]

When new rad valves are fitted you can use Fernox inhibitor gel, comes with tube to inject into radiator via vent plug.
Flush system before doing this and check theres no leaks or you will have to drain sytsem down and repair leak(s) resulting in loss of inhibitor.