Hi,
A quack update:
We let the mother duck and 11 ducklings (aged ~60 days) go down to the river last Saturday morning and it's been a success.
The ducklings had been doing short flights in the garden and the mum had been spending more time away, so we thought that they were big enough and that we needed to let them go before the mum abandoned them.
Now they are now all on the river. They seem happy and have managed two nights without getting eaten. They haven't been back to our house, though (it's about 20m from the river). I hope they are getting enough to eat - they have been so well fed by us that they are about 1/8th bigger than the mum!
Thanks for all the words of advice. We are now trying to restore our lawn
cheers, Luke