About the middle of July I noticed a self-seeded tomato plant, just a couple of inches tall, in amongst my root veg. Probably came from a seed that had been in the home-made compost. Anyway, the seedling looked nice and healthy, and I thought I recognised it as a Wilma (it has a very tight, compact structure, like Wilma has). I hadn't the heart to just rip it up and throw it away as if it were a weed, so I planted it in a spare container (notice another re-cycled container -- this one is a tub from pelleted chicken manure).
The plant has grown well and is still looking healthy. It has plenty of flowers on it now. Whether it will have time to produce ripe fruit before the Autumn frosts arrive I know not, but it's worth a try. Since the plant is never going to be huge, I can always put it inside one of my little plastic greenhouses for a while. Maybe I won't risk bringing it indoors though.
There you are then -- a good example of how all gardeners should be opportunists as well as planners!