I'm going to apply some more logic to Thomas Pateman's army service, based on my army service numbers database. Although I will be launching the armyservicenumbers.com website with infantry regiment service numbers, I do have considerable data for other branches of the British Army.
On Thomas Pateman's page I state, "I could not find Thomas on the 1901 census and it is quite possible that by this stage he had already joined the army and was serving overseas, possibly in South Africa." We know that Thomas was serving as an RSM with the 4th (Queen's Own) Hussars by October 1915 and that he was a pre-war regular soldier. His medal index card gives two numbers and it is the first, four digit number - 4582, which is important here.
Prior to 1907, the cavalry numbered individually by regiments. This changed in 1907 and they numbered by corps - so one series of numbers for the household cavalry, one for the dragoons, one for the hussars and one for the lancers.
Assuming that Thomas joined the 4th Hussars pre 1907, his number indicates an enlistment date of late 1899 or early 1900. If he enlisted after 1907, this number would have been issued to a man joining up with the Corps of Hussars around August 1909.
It is difficult to say with certainty when Thomas enlisted in the British Army, but given that he was already an RSM by 1915, my money would be on an enlistment in 1899/1900 and that he probably extended his army service and therefore was still serving with the 4th Hussars (rather than on the Reserve) when war was declared in August 1914.