It probably bears repetition that Denis built this loco in the wilds of northern Nigeria, where his only machine tool was a lathe & he had no access to foundry facilities. He couldn't even get Mig welding sticks so had to make his own when fabricating components that most folks would cast, such as cylinders, saddle, stretchers, etc.
John found that the poppet stem guides were rather worn &, even when replaced, had quite a leak rate so he introduced stem seals. Having no return mechanism, John introduced light return springs for the poppets.
One late surprise was that Denis had glued the front onto the centre cambox rather than machining from solid and the old glue had partially given way. John cleaned all the glue residue off & micro-Tigged the front back on and it's now oil-tight.