Aiptasiogeton pellucidus var comatus: this backlit view led to conclusive identification as first Aiptasiogeton pellucidus, and then variety comatus (as opposed to variety pellucidus which is well known outside the UK). This pic of its delicate beauty is what made me want to call it the Fairy Anemone. Only 6mm high, in fact! Has since grown to appx 15mm
Fairy Anemone 3