On Sunday morning I popped up to the site in ST1185 where Julian found the Aphanorrhegma patens. Sure enough it was widespread across the wetter, barer patches in the field shown in the photo below, along with abundant Riccia sorocarpa.
Various Persicaria were abundant and quite confusing. I could certainly see some plants of Small Water-pepper (P. minor) among the commoner species, but was unsure about Tasteless Water-pepper (P. mitis). Julian and Tim Rich think they have some good candidate material for the latter from this same field, but it requires confirmation.