If you thought you'd walked into a minefield with Richard Simpon's article then the bad news is that the article is out of date as the code has been revised and made tougher! The following dicussion on the Model Engineer website might give you an idea of what is happening LINK.
The other thing is that if the boiler is made from brass or has brass fittings then it will be regarded even more severly due to problems of de-zincification.
If you want to steam the boiler in private then you don't need to bother with the regs etc. but for your own safety get a pressure and steam check done. If you want to use the boiler in a "public" environment then you will need to make sure that you arecovered by insurance – either your own or through a club and the boiler will need to comply with the necessary regs. otherwise the insurance would be void.
It really does look at the moment that the way things are going live steam powered models are really not worth the hassle that goes with them. Unfortunately the very people that should be standing up for modellers seem to be just going along with what are becoming ever more stringent requirements, with it has to be said no real case being made for the tightening of the regs.
Keith