hi ron,
ive had to do this a number of times with badly maintained club locos. it is rather laborious if the boiler is on the loco.
pour soda crystals into boiler through a suitable opening such as safety valve bush using a plastic funnel. boil up quite a few kettle fulls of boiling water and add to boiler. slosh about a bit as much as possible. keep the regulator closed!
drain down with as great a flow as possible. open both boiler drain valves fully.
you probably need to do this 4 or 5 times, and then repeat with just boiling water 4 or 5 times. then do a couple of static/stationary tests/steam ups of the boiler draining down after each time.
the drained waste can be collected in a bucket each time and examined till it is perfectly clear of muck and oil.
old marine enginners will know a lot about this!
do not run the loco till all the muck/oil is removed otherwise you will may get uncontrollable priming and damage the loco.
i knew Gordon Howells quite well and have very fond memories of him. those who also knew Gordon would perhaps agree he didnt look after his locos as well as some of us do!
cheers,
julian