That doesn't answer the question. Min BP needs to be checked by locking the boiler in min output using test mode, not by slowing a tap down. One millibar out will make a considerable difference on one of these. Rough and ready tests are unhelpful in this type of situation. Asking your engineer to check the min heat input should give them a big hint!
Of course if you are trying to do this yourself you must realise that we cannot give out gas safety related advice.
Of course if you are trying to do this yourself you must realise that we cannot give out gas safety related advice.