Didn't know you were at the chalk-face. '85 was when I started supply teaching, so wasn't aware of a hike in salaries. When I was a regular teacher, the NUT were the left-leaning, strike-prone lot and the NASUWT were not. Unfortunately, even then, the theat of parent/pupil action etc. was strong, and one took a career chance without a backing union. I resisted for most of my years but finally caved in; can't even remember now which one got my patronage. Small schools, including mine, were closing at a rate of knots so things were a bit chaotic from around '1980 to '84 or so.
Strangely, despite supply teaching probably being more fraught with danger (kids like their regular teacher), I was never approached by a union rep. or contacted otherwise. Chalk under the bridge now, I'm happy to say. In my EFL career, I was never made aware of ANY union, yet we taught children as well as adults, and some of those rich Russian kids could be problematic !