”wont do your system any harm” i have been a big user of leak sealer and have had some amazing results.. HOWEVER.. do not use it if your boiler is still under manufacturers warranty as they will likely not honour it if they discover leak sealer in the system.. WB especially keen..
if out side of warranty..and unable to get replacement from fernox.. turn off the red taps either side.. drain the tf1 via the tap at the base.
remove upper lid remove metal valve at base of tf1 clean replace inc. boss white and ptfe. refit the upper lid, make sure rubber seal is in place and that you ‘lubricate’ it silicone grease .. refill and admire your macho DIY profficency .. or get wet..