Well I found it, conveniently concealed behind the fuel filter and its associated pipes.
I've a horrible feeling you are correct regarding the glow plug fault. The engine was just turning over with no attempt to catch. The old trick of pouring a kettle of boiling water over the block provided enough heat to get it going. Replacing the glow plugs will involve stripping down the top of the engine, air intake rocker covers etc which I did earlier in the year after the injector. sealing went tits up. There is no room at all to work under the bonnet, another payback for waterloo from our French neighbours.