The northern race of Ringed Plover (Tundra or Arctic Ringed Plover) has been recorded at many sites in Britain, sometimes in considerable numbers such as the 500 recently at Breydon Water in Norfolk. My own experience of them comes from birds seen in Norfolk in the '90's where we considered them to be regular if scarce migrants (Google 'Tundra Ringed Plover' for records). The problem with'psamamiscus' identification is that there isn't a nice neat cut off between this race and the familiar 'hiaticula'but a clinal change with intergrades between the two. However, a combination of appearance, feather wear and 'jizz' should nail most individuals, especially when the two races are present allowing for a comparison.
As well as being smaller and darker than 'normal' Ringed Plover, they are quicker in movement, giving the impression of a chick or maybe a Kentish Plover as they dash about. They can be really quite distinct, especially if compared to a lumbering, pale 'hiaticula' Ringed Plover.
Additionally, Tundra Ringed Plover has a different moult pattern to 'hiaticula'; unlike nominate birds they undergo a complete moult on wintering grounds - so fresh flight feathers in spring can be a helpful indication of 'psamamiscus' and worth looking for if you have an interesting Ringed Plover.
My interest in this form of Ringed Plover as a Pembs bird goes back about ten years to a record of three of them (with a 'hiaticula'bird for a neat contrast) on the beach at Fishguard. I think it was 2002. I sent the record in for the Report at the time but they 'disappeared'. (Graham Rees has kindly recovered this record it was actually two birds on 19th January 2013). Tundra Ringed Plovers can and do occur here in Pembs, probably regularly. The bird below was present at Fishguard today, found by Adrian Rogers, and being extremely confiding like many high arctic birds can be. The freshness of the wing feathers, apparent size and colour make it seem pretty convincing to me. (all photos Adrian Rogers).