Some good news, some bad ...

My God, but have I been having a duel with this beastly creature!
Paid the price too as have got rather nasty sunburn from lurking
around the pond all morning. Found that it was sticking to one end of
the pond but you weren't kidding when you said it was a fast, strong
swimmer! Quick as a flash. Tried to lure it out with a bierwurst-net
trap but only managed to attract flies.
However, found it hanging around one particular plant, thence
endgame. Sat there, like a destroyer waiting for a u-boat, until it
ran out of air and floated up to the surface. The first time it saw
me coming and went straight back down but it didn't have the sense to
swim away and, eventually, it came back up in the same place.
Best tip I can give is to watch where you cast the shadow of the
net. If it falls across the beetle it'll vamoose like a torpedo. By
coming at it from the same side as the sun I managed to scoop it out
and squish the little devil before it could fly away. (btw, in the
cold light of day it was only about an inch and a half long, not two
inches!)

However ... during the hunt I also came across (and disposed of) a
couple of beetle larvae. This is obviously not good. Apart from
anything else they are quite the most revolting looking horrors, more
like something out of Starship Troopers than a beetle. Fortunately I
have managed to account for all the fish so none of them have been
torn to shreds yet.
Cheers for the info and advice anyway, I will simply have to keep a
close eye on things for a while and hope that there's no eggs in there
....