Hi Stewpot,

There were two basic ranks of senior non-commissioned officers (SNCOs) in this period, namely Sergeant and Sergeant Major. Sergeant is sometimes spelled serjeant, the older form. The other ranks, colour sergeant and staff sergeant, started out as the titles of appointments. So for instance a colour sergeant would normally be the senior sergeant within an infantry company and he would usually be in charge of the stores and company equipment, a post which today would be known as company quartermaster sergeant. The Colour part of the title comes from the fact that the Colour Sergeants of a Regiment had the privilege of guarding the Regimental Colours in battle. Colours are the big flags which act as a rallying point for a regiment in chaos of battle, and would normally be carried by a junior officer (who would often hold the rank of ensign) close to the Commanding Officer or Battalion Headquarters on the battlefield. A staff sergeant was also an appointment with similar day-to-day functions, and was normally found a non-infantry units such as cavalry, artillery or engineer regiments, which did not have colours. 

The Sergeant Major was the most senior non-commissioned officer in a Battalion. Today we would refer to him as the Regimental Sergeant Major or RSM.

I have never come across the title of sergeant first class in the British Army.

As for their historical status, Sergeant (or Serjeant) is a very old rank going back to medieval times which is why you find it in many other contexts apart from the Army, such as the police. The colour sergeant came into being sometime around the early 1700s when Colours were first seen in their current form; prior to that the emblems a regiment took into battle were often the arms of the colonel who raised the regiment. The formal rank of colour sergeant was created after the Napoleonic Wars. I don't know when the rank of sergeant major was first created, but it also pretty old. All of this changed around 1913 with the introduction of the rank of Warrant Officer. The most senior SNCO in a company became a warrant officer class 2, and was referred to as the company sergeant major, while the rank of staff sergeant was retained between sergeant and warrant officer, although the infantry (and Royal Marines) still use the name colour sergeant for this rank. At the same time, the old rank of sergeant major became a warrant officer class one, and soldiers of this rank might hold a number of appointments, including that of the regimental sergeant major. Until 2015 the most senior warrant officer in the British Army was actually the post of Conductor in the Royal Logistics Corps, but now the most senior post holder is the Army Sergeant Major.