Just for a bit of an update on this...
I went up to the loch (and the smaller one beside it) last night for a look...sat and watched the water for a good while and it was like glass - completely flat like a mirror. Anyway, with the conditions you would have been able to spot any fish activity a mile off, but there was absolutely nothing doing.
Surprising as well as the fly/insect life was incredible! Any fish in there would have had an all-you-can-eat buffet...so I'm wondering if it's either been fished til it was empty - or if the local birds have maybe had all of the fish for their dinners too.