This was the only clump of frog spawn near the edge of the pond that was completely submerged and free floating. The other clumps of spawn, easily accessible from the vegetation had been laid partly on the vegetation and stuck fast (I tried to move them into the water, but could not dislodge them without fear of damaging the eggs).
Any ideas! My thought was a fox; we do have foxes that pass through our garden late evening and early morning. We also have lots of cats around this way and they do take a lot of interest in the pond, but do little more than dip their paw in - and not really enjoying the sensation. We also have lots of herring gull, but I can't imagine them being able to drag a clump of spawn out either.