I thank you for the point, but I felt unsure, when I posted my answer to you.
It is a Rational 10 HP for sure, but most of the sources say 'Heatley-Gresham Engineering Co. Ltd.', Royston (Hertfordshire) is the manufacturer...
Is 'Ralph E. Sanders & Sons', Hitchin, Hertfortshire, UK, only the coachbuilder?
Two pictures attached