Only one way to do it properly .
Remove boiler from fire box .
Remove end cap clean all solder and resolder .
To remove the endcap i wrap the boiler in a wet but wrung out tea towel leaving the endcap and an inch or so of boiler exposed ..
I use a vertical gas bottle and torch and rotate the endcap around in the flame .
I use the back 0f a knife and slide the knife gently along the boiler and tap my way around the endcap untill,it come off . At which point it will roll 0f the bench and under the fridge . The wife will come in and demand to know why her tea towel is scorched . Then she will notice the smell emanating from under the fridge from the hot endcap and dust .
Cheers
Dennis