Hi Penny,
This page has the estate walks (which is the bit you need) as being open dawn to dusk
http://www.nationaltrust.org.uk/main/w-vh/w-visits/w-findaplace/w-felbrigghallgardenandpark/
The donkey paddock mentioned on the pager messages is to the right of the hall, as you face it from the car park and has ..... umm, donkeys in it as a slight clue! The trees between there and the car park were where we had the Hawfinches but admittedly that was on Jan 1st.
For the lake, stand with your back to the hall and look south. You can take any of the paths leading south from the car park to reach the lake. I tend to follow the path leading towards the church over to the left, go through the gate and then go half right on another path towards a wall (the lake is hidden in a dip in the ground to the right of this wall). Sorry .... sounds very complicated but in practice, I'm sure lots of people will be heading this way and I 'think' there are maps in the car park of the layout of the estate.
I've also had Grey Wagtail at the lake and don't forget the Little Owl in the ivy covered tree which I apparently managed to walk right past last time! Not sure which ivy covered tree, mind you...
Good luck
Irene