Hi I have been having same problem since boiler was 6month old,turn hot tap on or ch and boiler keeps starting and stopping, have found by replacing the hot water and central heating senser's or Thermister as they are known part number 0007ter05005/0 cures the problem of boiler cycling between fireing up for 2min then shutting off,Thermisters are about £12 each on ebay.
drainman, January 2009