Hi Oz,
A good way to clean very dirty guns is to take it to bits,then put all the parts into a saucepan of boilng water with washing soda in it(I tend to do this when the wifes out).Bring it back to the boil and simmer for 20 to 25 minutes.Put wire on the large bits so you can handle them as they'll be hot when you take them out.I normally put the small bits in half a coke can with loads of holes punched in it so it'll drain when removed.As you remove them they will dry instantly.As they dry spray them in oil such as youngs 303 or similar.This process removes years of crud from every nook and cranny and bring the remains of any colour hardening back up again.I normally reassemble the gun using vaseline with a small amount of oil melted together,then left to reharden,as a lubricant grease.This way of doing a strip and clean is suitable for any old and most new rifles and shotguns.
Pugwash