What boiler is in the property? A mm odern condensing boiler can rust very quickly, also did you have a gas safety check only recently or an annual service aswell?
Easy enough, isolate flow and return and crack the nut on the end of the return iso, the gauze is in there, check in the manual for detailed instructions and location
Bravo for holding your hands up, most here would bullshit their way out of it, we can't know everything
. Sick to death of know it all's on here that can do no wrong, making others poster wary of looking a fool or being shot down.
Rant over!