Hello Rosalind,
I was really excited to come across (albeit rather late) your quest for information regarding Sassoon Joseph Sassoon. I have in my possession a toleware military 'hat box' on which there are two brass plaques. One reads Jones, Chalk and Dawson, 6 Sackville St London and the other S J Sassoon, 6th Inniskilling Dragoons.
I contacted a Mr Keith Miller from the National Army Museum who kindly researched the name and replied: "The hatbox would have been for the dress helmet of an officer in the 6th Inniskilling Dragoons. It would have been supplied by the firm of Jones,Chalk and Dawson who were one of many firms of tailors who supplied uniforms in the early part of the twentieth century – the firm is still trading today.These uniform items were only supplied to officers who had to purchase their own uniforms and weapons. Sassoon would have purchased the helmet when he joined the regiment.
The rather strangely named Sassoon Joseph Sassoon joined the regiment in Feb 1907 as a Second Lieutenant,was promoted to Lieutenant in Oct 1908 and Captain in Feb 1913. He served throughout the First World War but was seconded to the War Office from Feb 1916 until he left the army in Dec 1919."
I hope this is of some interest to you.
Regards, surreyman