I think you will find the case on this boiler is the seal for the combustion chamber and therefore would come under the label of 'gas work' and certainly replacing the thermocouple would do. You must by law have a CORGI registered engineer carry out this work. By you title I am guessing you could be a landlord or agent or simular and doing jobs like this can have a prison sentence and or a severe fine attached.
toots
January 2009