What is wrong with my Ikon boiler?

I hyave an Ikon boiler (the identification lable is inside the boiler so I'm not sure of the model as I am unable to read it as it is too far inside for me to see) for my central heating only. It has automatic lighting for the pilot and last week it broke down with the boiler clicking in an attempt to relight the boiler and the fan starting to kick in, the fan would stop and the clicking would start again. I eventually got BG to come out and they replaced the control panel and the fan. This ran ok for 3 days and has now gone again and is doing exactly the same as it was before. I have booked for a BG engineer to come again on saturday (today is Wed) but I am wondering what could cause this kind of problem to keep happening. The engineer did say that at some point my system would need flushing as there was debris in the pipework, (proved to me by the copper pipes being magnetic ???) telling me that this was expensive but at no point indecting that this was the cause of my current problem. Other than that I was given no indication of what could or had caused the problem.