So I have now divided my rhubarb. Actually I should have done this some time back, during the winter, but seeing as we haven't really had much of a winter and the plant has been growing pretty much throughout, I don't think it's much of a problem. Having not done this before, I was in for an interesting time.
The main thing I hadn't realised was just how much of the plant there would be underground. I found a fibrous mass a bit larger than a football (but a lot heavier) and a number of cable-like roots heading off in all directions, including downwards. Basically, there was no way I was shifting that beast. With excavations all around the plant I decided to get heavy and shove the spade in to split the root/stem mass were it sat. A little later I had four hefty new crowns, two of which I replanted right away (the whole point of this is so in future I can force one plant for an early crop and let the other develop naturally), one I gave to a neighbour, and another is promised to friends on the other side of town. I was also left with a small piece which I have potted up for the time being, pending some better idea of what to do with it.
Now only time will tell if I have managed to kill off my lovely rhubarb plant or if I have improved the productivity of my garden and also made some other people happy. The whole process felt more destructive than anything, but I get the impression that rhubarb is one of those resilient plants which are hard to kill. Fingers crossed...