I have answered my own question and for anyone that is interested, found that we can camp at the Loch Insh Outdoor Centre. They don't advertise camping but from a telephone conversation today they do allow single night backpacking stays on the Loch Shore. - £6 per night with W/C's & Shower.
I've camped a short distance by the River in the woods on the west bank of the Feshie just a short distance down from Feshiebridge opposite the tail end of a river Island.
At the head end of the island, I e seen people camping there- when the water is low there isn't any water flowing down the west channel.
And also on the west bank there is an open green space of grass too.