Thankyou for all your replies.......I've looked at the links provided and found them helpful as well........have had no other offerings yet........
Have added extra bells to his collar so I can hear him if he turns over in his sleep
I'm also going to get one of those ultra sonic cat deterrants and put it by the tree area to try to protect the nests.
I'm not going to harness my cat. But I will do all that I can otherwise to protect the local wildlife. He's taken quite well to being allowed outside just during the day........although he still cries to go out at dusk.........he can see the birds feeding from his position on the windowsill.........
A friend suggested bramble clipping put round the base of the trees to make his path more difficult, which I will do as I've just hacked my way through 18' of them.........the previous occupants didn't like gardening