It is not a common plant nationally - see the excellent BSBI Database which now has open access to the maps, and perhaps use it to identify historic locations for targeted searches. I am sure it must still be present at most of the larger upland lakes and reservoirs. Following on from the last blog it has now been found for the first time at Eccup Reservoir (two new tetrads and one new hectad), Scar House Reservoir (new hectad) and Scargill Reservoir (first hectad record for decades). Please bear in mind that not all these sites have public access, there are plenty more reservoirs with public access worth searching. I would also love it if it was found in Huntingdonshire, where there are plenty of old mineral workings. It is much less common in that part of Britain, but it is known from similar habitats at Titchwell (VC32) so its not impossible and there are a number of very old records.
Part of the reason for recent finds is focussed searching at a time of year when botanical recording activities are winding down. I have been lucky enough to be doing reservoir surveys recently and have been getting my colleagues to keep their eyes peeled also.
Other associated highlights include a new population, and a large one at that, of Small Water-pepper (Persicaria minor) at Eccup. This a major range extension. Like the plants in the reservoirs along the Washburn valley, the plants are all white flowered. Eccup also provided 1000's of plants, enough to be certain it was not just a chance aberration, of an unusual form of Red Goosefoot (Chenopodium rubrum). This goes by the name of var. pseudobotryoides and is readily distinguised because the plants form prostrate to decumbent mats with small leaves. The stems upturn at the end where they produce short inflorescences. Stupidly, I took no photos but there is a picture here. Not the best botanical illustration but it is excellent for showing the general jizz of this variety.