Phil … try a pressure sprayer from JACKSONS MINIATURES in Gillingham … web- site is http://www.jacksonsminiatures.com … called a SUPERIOR water top-up bottle … 2 sizes available.
The silicone tubing might be a special type … at the moment I am using ordinary fuel tubing available from the local model shop … so far it seems to work … my own home-made valve is outside the boiler, so the tubing is not sizzling merrily INSIDE the boiler … I assume there is an exact size for the metal valve middle to match any particular size of silicone tubing … hopefully I will discover if there is an optimum size, but I am also learning about railways.
I was recommended to try a large size garden pressure sprayer with a capacity of 1. 5 litres … not much use because it ran out of pressure too quickly.
hope some of this may be useful … martin