Lister STW2 20hp starting problem

[ColC]

My Lister diesel is about 70yrs old, but it has been running fine until recently. My batteries and starter motor had reached their use by dates. I replaced them both. When I turned it over with new parts, it would not fire, the motor turned over without a problem. Vapour was coming out the air inlet, so I am assuming fuel is flowing OK. The fuel had been in the tank for some time, so replaced the fuel and drained the fuel lines. I checked the decompression mechanism and it was fine. I am at a loss as to what the problem could be. It operated without any problem on my last trip about 6 weeks ago. It was not lacking any power at the time. It has never had a problem starting for last 11 years I have owned the boat. Does anybody have an idea what could be preventing the motor from firing?