Noting you are from the UK, across
the pond.

What I'd do is brew up some coffee tonight
and head out to the pond well after dark
with a flashlight. If you are not seeing anybody
chowing down on the plants during the
day, chances are they are out there in the
dark hours.

If you can catch one, bring it into the house in
a jar full of pond water and some plant matter.
Then you can cruise the internet and see if you
can ID the fellow.

Around here you can call the Master Gardeners
or the University Extension office and ask them for
help. I'm betting in the UK, the land of lush gardens
you'll have some resources like that.

kathy :-)
www.blogfromthebog.com