Answer:
On the OS Explorer series (Scale 1:25,000) of Ordnance Survey maps, a well is denoted as a small unfilled blue circle with a blue capital "W" next to it. Springs are denoted by the same circle however the letters "Spr" are used in place of "W".
The smaller scale maps (i.e. showing a larger area) such as the OS Landranger series (Scale 1:50,000) do not appear detailed enough to display wells (and they do not appear in the Key).
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