The message
from Gwenhyffar Milgi contains these words:

No, not an elver. At least it didn't look anything like the pictures I
found.

Oh great. Now I'm going to have to lay-about in the mud with my
camera!

Yes you are, because we all want to know.

4 cm's long, glittery grey/silver in colour,
and it was definitely an amphibian. It had no legs.

Only other thing I can think of is a very young slow worm, or possibly
a baby grass snake. Are they that tiny? Dunno. Snakes can (and do) swim
astonishingly well, but I don't know if slow worms can.

Janet.