Heres the way I do it.
15KG bottle of gas (blue one, as the gas in them is heavier and will sink).
Dig a hole under the stump and run the tube from the bottle under the stump.
Give it a minute or two depending how big the stump is.
Turn the gas off. Take the tube out of the hole and plug it.
Put the bottle a good way away from the stump.(at least 20 yards).
Make a petrol bomb and light it. Throw it at the stump and don't forget to run.
It works for rats and wasps under sheds as well, but you only need about 30 seconds of gas or your shed gets trashed. (Did that once and leant a lesson)
Oh! and don't smoke while your doing it.