To be fair, Potterton BB's were not the best. One annoyance was the flueeways are relatively small and easy to soot up. You say your guy is a competant heating engineer. Is he GS registered? Did he physically brush the boiler heat exchnger THROUGH the narrow flueways vertically and horizontally. No disrespect intended, but you'll understand that we have all seen hoovering and servicing to lax standards. Added to that, a lot of youger heating fitters have no experience of BBoilers. Your symptons still suggest a dirty boiler