Finally identified a plant that has been growing in my garden for several years. It had been introduced almost certainly as a soil contaminant. The plant in question is New Zealand Bitter-cress. There are hundreds of plants in my garden and it is growing between the cracks in my patio. The plant is tiny, rarely more than an inch high with tiny typical bitter-cress leaves and rarely more than an inch and a half  wide. Unlike the extremely common garden weed Hairy Bitter-cress it only has a single flower on each stalk.The single flower is much larger than Hairy Bitter-cress despite the whole plant being only a fraction the size. It is not mapped within the main volume of the New Atlas of the British and Irish Fauna, but is shown on the accompanying CD. There are only 13 other 10 km sites shown on the map, none remotely near Gtr Mcr. However I suspect that it is being massively under recorded.I have submitted the record to RODIS.

Due to it being tiny although numerous it is causing little or no problem. However for anyone growing the tinier and more delicate alpine species on raised beds, it could well become a problem species.