One product of this development was the 1904 Siddeley known as the "Green Goddess". The curved and slightly pointed radiator shell was to become the characteristic of the Wolseley radiator until the late twenties.

In 1905, Wolseley merged with Siddeley Autocar to form the Wolseley-Siddeley Motor Co. Siddeley became Sales Manager, then Works Manager when Herbert Austin resigned in 1906. An immediate response was to produce a Wolseley-Siddeley logo in "Art Nouveau" style in the company's advertising material. The name Siddeley Autocar became more and more prominent, Siddeley even being featured in his bowler hat in an advertisement for a car in an exhaustive trial.

When Tom Vickers died in 1908, his brother Albert took over and immediately pressed for a new approach to running the business, demanding a dividend for investors. As a result, less money was ploughed back into expansion and Siddeley resigned along with his main backer, Lionel de Rothschild and most of the design team. Wolseley, a great Edwardian company, then a serious rival to Rolls-Royce and Daimler, declined and was later to be absorbed into the Morris empire.

The Deasy Motor Car Manufacturing Company was formed in February 1906 by Captain Henry Hugh Peter Deasy. Born in Dublin in 1866, Deasy joined the army in 1888 and retired in 1897. He won The Royal Geographical Society Gold Medal in 1900 for surveying nearly 40,000 square miles of the Himalayas. Deasy, like Siddeley, was an entrepreneur, and in 1903 helped promote the Rochet Schneider Company by driving a car from London to Glasgow non-stop. He also drove a Martini car up a mountain rock railway near Montreaux, Switzerland.

At this time H H P Deasy and Co., was formed to import both Rochet-Schneider and Martini cars into the UK. The Deasy Motor Co. took over the factory used by the old Iden Car Co. at Parkside, Coventry. The vehicle designer was Edmund Lewis of Rover and Daimler fame. From the outset Deasy and Lewis were at loggerheads over the way things should progress, the main problem being the lack of urgency in getting vehicles to customers. There was no doubt that the company was undercapitalised and paying the wages on a Friday was always difficult. Nearly all the components in the vehicles were bought in, but the machining was of indifferent quality and much re-working was required, smoothness being at first sacrificed for expediency. Deasy became increasingly frustrated with this state of things and resigned on 9th March 1908. Lewis was relegated to Consulting Engineer.