At around 1730 they were the dominant species over Marloes Mere, with 25+ perched on the phone lines on the eastern side at any one time, and several times that many in the air, along with much smaller numbers of Swallows, and a handful of House Martins.
I got an impression that at least some these might have been newly in: there was a steady stream coming in over the fields from the direction of Marloes Sands. But, of course, they may have been flying in circuits! It was all still a lovely spectacle: I don’t think I have ever seen this many Sand Martins in Pembrokeshire.
Later in the evening a smaller group of perhaps 20 Sand Martins fed briefly over Martin’s Haven before moving on (hopefully to somewhere warmer!).