I hate the word heating engineer and gas engineer, it's a plumber and those jobs, heating etc is what plumbers do. Go design a heating system for a factory or a big hotel, then you can call yourself one

Probably no need for flushing the system thats a couple of hundred quid by itself lol well few hundred with british gas

So highly doubt he'll just get his powerflusher out and give you one. If it has been re-filled recently ie drained air let in, then could be an air lock, or it may just need balancing. Doubt it's nothing that serious.