This "Simplex" came in recently as a part exchange, an older engine that has recently been repainted - the boiler was removed at this time and, with cladding removed, had a new hydraulic test performed by the owner's club.
Silver solder copper boiler, 90psi working pressure, feed by axle pump and hand pump - the previous owner gave up trying to get the original injector to work and removed it, I would be tempted to put one back on. Cast iron cylinders with slide valves actuated by Walschaert's gear, screw reverser in cab, mechanical lubricator.
I took it out to the club last weekend and ran it round for the afternoon. Essentially sound, it needs a degree of fettling, including refitting a smokebox door which doesn't close as tightly as I'd like, to the detriment of the draughting.