Thank you for your very useful comments.
It has P.O. Brush Code labels on the alternator and exciter, so I had assumed that it was probably from an exchange.
If there is a mains immersion heater, it would explain the junction box labelled 240 Volts.
The solenoid stop is seized solid but apart from that, things are in reasonable condition and the exciter commutator shows very little sign of wear.
From the way it is plumbed in, I had guessed that the oil tank was part of a dry sump system but your suggestion of an automatic top-up arrangement makes more sense.
Unfortunately someone has cut off and thrown away the electrical control panel!
I will drag it out of the gloomy shed and start work on it.
The owner wants to install it for occasional use at an isolated farm, on the basis that people will be less likely to steal it than a more modern (and much lighter) genset, especially if we fix it down with rag bolts!
Chris