hi martin, it could well be the heat exchanger causing this problem!! the flame shouldn't modulate, the working pressure for a domestic house system should be 21MLB, give or take 2 mlb which basiclally means the flame should stay constant. a suggestion is to call the gas board on 0800 111 999 and say that you have pressure problems. they should send someone out within the hour. i have been called out to a couple of jobs just like this one and its been the regulator on the inlet to the meter that has been faulty. the correct pressure in a house is 21millibar (+/- 2mlb) coming out of the regulator. however, if the pressure is too low the flame will sit low on the burner.
hope this helps
Stu
April 2008