Hi Tony, what you have is a Coventry climax FSM pump, with an engine that was made by Coventry Climax themselves. These engines were designed in the 20s or early 30s for a car manufacturer called Swift Motors( the engine number,when you find it will be SM.xxxx)
Swift motors went bust, so Coventry Climax incorporated the engines in there own pumps. They built many of these pump sets before and during the war. The good thing is that you have the trailer, they are harder to find.
I have 2 of there pumps, one Ive just restored from a pile of bits like yours, and another waiting in store. Also working on a Beresford Stork at moment and im picking up a Worthington Simpson pump and trailer on Monday!
All these little trailer fire pumps were ordered before the war for the AFS & then incorporated into the NFS, some also saw military service, they were towed by commandeered private vehicles at first until proper tow vehicles became available later in the war.