Here is a map from Google of how to get to the roost.
From Morrisons I followed the main road through Staveley and then turnedright down Fan Road which becomes the A6192. Follow this road as far as the water and look for a car park on the right.
This Google Earth map gives a bit more detail
We had to walk from here along the grass verge of the A6192 until we could turn down a footpath to the disused railway. I think it is possible to do this without having to walk on the grass verge. Once you have got onto the old track follow it towards the Motorway and look for a square pond on your left. The starlings usually roost at the back of this pond. It was the best murmuration I have seen for some time!
If you are early enough you may be able to fit in a small car park on the main road a lot nearer to the roost site and then just walk down a path for about 100 yards to the railway track.
Hope this helps!