It appears that S&D were motivated to enter the municipal fire engine business because Dennis was eating their lunch in the dustbin lorry (garbage truck) business.
This was a "tit for tat" tactic, which ended up driving both companies into a merger because neither of them could remain profitable.
In entering the market S&D decided to ape Dennis, and even used the same engines and components, including a cab which was designed to look almost identical to the contemporary Dennis.
They did not innovate, or do anything meaningful to improve on the Dennis products, but they sharpened their pencils and cut prices, which is why they managed to take so many sales from Dennis, including the two large batches of DPs for London, which included this example...
I've read that Ogle Design were contracted by both companies to design a new cab unbeknown to each other and Ogle saved a lot of money by giving them both near enough the same cab !
Added by CraigW on 13 March 2018.
Very interesting, but do you know, I've never once thought that S&D fire trucks looked like Dennis ones. It never once occurred to me. Even now, after having their shared roots pointed out to me, I still don't see it. Very similar, yes, in that they are both rather plain and practical, utilitarian if you like. But if you gave me a pen and pencil and asked me to design a fire engine cab with two doors, with a window between, and two steps for the lickle fiemen's legs to climb up, my design may well look like a long-lost relative to these supposed brothers. And you'd probably struggle to work out wether my effort was a Shelvoke or a Dennis. But it would be neither; because it would be sporting a YBH badge!
Also - who built the Shelvoke cab? It looks lot like the Dennis, but I am not sure which came first?
Added by Rob Johnson on 27 April 2017.