Thanks for the information alan. With this I managed to find another post on the forum dealing with a different sprinkler but showing an advert for this one. Obviously I was wrong thinking it was a pump where it is apparently a sprinkler.
Geoff the central arm move side to side and has a toothed rack on it. This connects with a toothed gear below that actually makes the output shaft (photo 2) rotate side to side. I guess this is where the sprinkler head fits. I will take a couple more photos when I am in the shed next weekend.
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