I had a jolly trip to Broad Haven this morning since there was a very low tide at a convenient time -- and although the temperature was just above freezing, with rain and sleet and a high wind, mission accomplished. The big boulder of "columnar blue quartz-porphyry" discovered by Cantrill et al a century ago is still there, and is still washed by the tides every day. It's the size of a small caravan -- by far the biggest erratic I have ever seen in Pembrokeshire. It's got a very irregular shape, and is not at all smoothed or rounded off by either glacial transport or wave action. It's difficult to estimate its volume, particularly in the middle of a deluge, but I would say it's at least 27 cubic metres -- which would make its weight about 75 tonnes. (For comparison, the Freshwater Gut erratic is said to weigh about 50 tonnes......).
Next to it is a smaller boulder with an estimated weight of about 22 tonnes. It's made of the same material, as far as I can see -- so is probably from the same source. Maybe it has broken off the larger chunk of rock during or after glacial transport. The colouring is dark blue, but there is heavy weathering on the rock surface.
That's not all. I found another three giant erratics within 50 m of the ones pictured above, and the satellite image below has been annotated to show their positions.