Well I am able to approach the issue reasonably logically, and follow a flowchart:) However I am not most practical person when it comes to DIY. If it comes to flushing the system I will find a professional.
I noticed that about three weeks ago the radiators were not getting hot, and so the house was a bit cool. If I left the Honeywell boiler timer control set to constant then the house would eventually warm up. (The timer is working OK). The boiler is a Potterton Suprima circa 1997 and is firing up...