Solved Could someone please identify this small plant for me?

Most likely just willowherb grown from seed so. Just to complicate things there are a few different varieties of willowherb that are fairly widespread. I have mainly two types in my garden and they both can form underground stems but one produces much thicker stems and grows bigger overall. I find the best thing is to be a bit gentle in pulling out the small ones from the ground and if they lift easily it is probably one with no underground stem. If there is more resistance than I would do a little digging about the little plant to loosen the soil before pulling on it again and this usually is enough to dislodge the underground stem which can lead back to another willowherb and help pull out the full colony produced from one plant. The size of the plant is not a great guide as to what to expect as one sprouting from a spreading underground stem can be small just like a seedling. If I do notice an underground stem breaking I can usually dig about where it breaks and find the rest of the plant as well. They are so good at spreading I have got plenty of chances to practice how to dig them out.

I sometime leave the bigger type grow if it is not competing too much with something else as the flowers are nice and more long lasting than the smaller type that seems to almost go straight to seed. I think I read somewhere that willowherbs were becoming more widespread in recent years and don't remember them being so much of an issue in the garden a few decades back. I think the article I was reading about them suggested that earthworks related to motorway development in Ireland had allowed avenues for them to spread.

 

I'd guess that one is Rosebay Willowherb, which I find quite attractive and it's also a valuable food plant for catterpillars of some moths (such as Elephant Hawk Moth), but there's only a few clumps of them growing locally.

But I've only seen one butterfly (Green-veined White) on Great Willowherb flowers in several years, and I've only seen them and other Whites pictured on Great Willowherb, but Rosebay Willowherb flowers attract a much greater range of butterflies and moths.