Be really careful with this - my daughter's partner did exacty this
including switching off all flames, including the boiler pilot light -
but the whole lot exploded on him when he applied the filler round
pipes as you were describing. There did not appear to be a igniting
source so it may have been a dust explosion - he was fortunate to lose
a bit of hair, eyebrows, etc. But the flame front was big enough to
set fire to the curtains.

Don't know what to suggest other than plenty of ventilation.

Rob