terry's been living in are lake for about 20 year's
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You have the right species, although I would say this is a red-eared terrapin Trachemys scripta elegans rather than a yellow-bellied slider Trachemys scripta scripta.
It might be best to move this to amphibians and reptiles using the edit tab.
Lat/Lng: 51.23236, -0.3511
OS grid ref: TQ152494