This coltsfoot (Tussilago farfara) appeared in one of the containers. There has never been one in the garden, although they are common enough round about, especially where earth has been turned over as part of building works in the next estate.
Since the flowers appear before the leaves, it must have been there last year (or longer!) and I didn't just notice it. This prompted me to look it up and I found this interesting coltsfoot page.
It seems that they grow well on clay, so it is ironic that it is growing in the container with compost instead of the heavy clay in the rest of the garden.