[Closed] Plumbers / Heating Engineers, will my pipes melt?

Random question for the day,

I had a plumber out about three weeks ago to look at my combi boiler that is struggling to heat water beyond just slow flow rate. He cleaned the particle filter, that didn't help. He came back as said he would flush the system and added in Fernox F5 express cleaner. He said he would need to flush this out after a few days but I have not been able to gt him to come back and the stuff has now been in the system for about 2 weeks. Is this a problem, will it cause damage?

I will need to get someone else out to sort the original problem but I am trying to work out how urgent this is.

Ta