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The 2001 Atlas of Scottish Water Beetles (Garth Foster for SNH) says under Gyrinus substriatus: 'The commonest whirligig beetle in Scotland, not to be confused with G. natator L., which has not been recorded reliably from Scotland and is extinct in England'. So you are probably right about it not being natator, but perhaps for the wrong reasons!
recording wildlife with The Recorder's Year on www.hbrg.org.uk/TRY.html.
I remember reading pretty much the same thing in British Wildlife. I think it still survives in the peat bogs of Ireland. But it is the species most commonly illustrated in insect guides so often gets recorded in error! For example Collins "Complete British Insects" (not complete at all!)shows its distribution as all over the UK!
Gyrinus natator was synonomised with substriatus until relatively recently (1980s). as natator is the older name that was what the combined species was called and many field guides have not noticed. http://www.jncc.gov.uk/page-5488 for account of natator in Britain and here for natator in Ireland http://www.npws.ie/en/media/Media,6698,en.pdf. You can tell them apart in the field but only from underside and side and to be honest with great difficulty.
Shady Whirligig is the name devised for natator to reflect its habitat and taxonomic history.
as to what the species is, the main characters are the shape, underside colour, claw colour and in the genitalia
Lat/Lng: 51.3, -2.7
OS grid ref: ST5662