Definitions:
ECWS: The English Civil War Society
KAotW:The King's Army of the West, later to be the
KA: The King's Army
RA: The Roundhead Association
SK: The Sealed Knot

A Brief History of English Civil War Re-enactment.

The Sealed Knot was the first of the societies and was indeed formed by "The Brig". That is, Brigadier Peter Young. at that time a noted Civil War historian. (Also, in his time, ex-commando, youngest Brigadier in the British Army, and Sandhurst lecturer.). The story goes that the idea came at a garden party and the first re-enactment was at Edgehill. In my time the birth of the SK was refered to as "The Edgehill Garden Party".

The next bit of the story is closely tied to personalities and internal society politics so I'll try to be brief. I'm no longer involve in re-enactmant, I don't have an axe to grind, and I don't know or much care what the current organisations are like.
Sometime in the early 70's some of the membership of the SK became dissatisfied with the organisation of the SK. This is the heart of it. There was an "Inner Council" which ran the SK. This council was elected by "full members" of the SK. The Inner Council selected those people eligible for full membership. The nett result being the paying members had zero control over the society they had joined and resented the fact.

The result was the KAotW formed by some of those upset by the way things were. They were from the south west of England, hence the original name. They were later joined by more dissatisfied SK members and became the more simply named KA. In 1974 when I became active in re-enactment the KA was a small re-enactment group, most members still held SK membership as well as KA, the common enemy was the RA which had already formed but I don't know the story of their genesis. Relationships between SK & KA where worsening badly at this time and within a year of my first event they were effectivley "at war" neither supporting or attending the others events. By this time (~1976) the KA was well established however and was getting on well with the RA who had more or less stopped supporting the SK events. This continued for some years...

Here I take issue with Dai. The ECWS was formed by the incorporation of the RA and the KA into one "limited company/registered charity" to provide an overall framework for events. It had an effective membership equivalent the the sum of the RA & KA. At this time the whole re-enactment thing was under much closer police scrutiny and control, particularly in the matter of weapons.
(Basically someone wrote a novel propounding a "perfect crime" by having a murderer use a black powder weapon at a military re-enactment - who do you blame when there are 200 similar weapons in a small field ?)
Things were still much like this when I finally gave up. The last event I attended being in 1986. I got tired of hard floors, wet fields and gunpowder farts.

NB The Great Rebellion Society see the other thread weren't really splitters. They were just a group of about 30 people who lasted about two seasons and the disappeared of joined the RA.
(Dai, were you thinking of them ?).

AndyG