You can charge for your time. They have powers to do what they need to, but not for free.I feel your pain.
I am going through a similar thing with Anglian Water.... sadly it appears that I'm on their preferred route for a new pipeline! For over a year now I have been plagued with phone call asking for access for a ridiculous number of bloody surveys which seem to involve various hippies in hivis wondering around aimlessly with ipads. Had surveys for Bats ...Badgers ....Stone curlews ... Otters ...more Bats ....more Birds and on and on.
The only thing they haven't surveyed yet is my underpants.
Never thought of charging them for every phone call but its now on my list of things to do....