Well, Dick; if you are already sufficiently informed to pronounce upon the accuracy or otherwise of my guess (you have, after all, made plenty of your own), then you didn't really need to start the thread, did you? I'd suggest that you contact the relevant organisations directly if you want a detailed breakdown of their membership figures over the last 20 years. I have provided links to their websites; you can find contact details there. Do let us know what they have to say, won't you. It would be mildly interesting, though whether it would be as gratifyingly controversial as you seem to hope remains to be seen.

Meanwhile, all three websites have lists of current member and associate teams that you can look at if you are interested. That won't tell you much about the state of play 20 years ago, when many of those teams didn't exist, but you may find it easier to start at the present and work back rather than the other way around. Remember that a lot of teams are started up by young enthusiasts and fold after a few years as work and family responsibilities (not to mention health and fitness issues) intervene, so a proper statistical analysis will require quite a lot of work. I'm sure that it would be a useful exercise if you are up for it.