Bats in London

Near the bank of the River Thames at Richmond one evening we watched bats in the dusk.  There were two distinct types; one was small, about the same size as I’d “expect” UK bats to be, having seen plenty in country lanes on evening walks before.  But the other type was much larger in comparison – probably double the size, though I can only give a relative measurement as it was too dark to guess a measurement properly.

Would anyone be able to hazard a guess about the larger type of bat?