It had seeded itself in one of the hanging baskets that are attached around the bird table, so my guess was that it was produced from seed out the wild bird mix. Thankfully, it wasn't hemp!
Several suggestions were made as to its identity, with the most likely being some type of red stemmed buckwheat. This, indeed, is what the mystery plant has turned out to be.
Now that the flowers are beginning to die back, you can clearly see the buckwheat forming. I've never seen this plant before now, so still haven't a clue where it came from or how it got here: there doesn't seem to be any buckwheat in the wild bird mix I'm using. I'll save the seed and replant it all next year, as the plant looks rather pretty with its tall, slender red stems and clusters of pale pink and white flowers.
After the windy, wet weather that resulted from the tail end of Hurricane Katia, it was great to see the sun shining and the temperature soaring today. By lunchtime, it had reached a balmy 24C here in Frugaldom, so it was definite T-shirt weather and perfect for getting out into the garden.