Tony, youre a plumber right, but unless youre a psychic plumber im guessing you wont be aware of the of the seemingly endless list of view forming problems this boiler has had since day dot. You also wont be aware of the length of time the tap has been dripping, and as you dont know the owner...
A full 11 years later and your post has just helped me out. I was all set to pull the diverter valve when I read your comment about a dripping tap and explanation of the problem. Which is the problem I have intermittently. Have just had the boiler do as you describe (short burn / long cool down...