Where does this 75% come from? for a residential property there is nothing to stop you having a webcam pointing out your window at another garden in the data protection act. The human rights act and "The Right to Respect for Private and Family Life, Home and Correspondence" would enable action to be taken - but there are no specific guidelines regarding this issue and no mention of a 75% threshold before action can be taken.
Asking about the webcam would be the first step - whether you put a letter through the door, knock and ask or invite them round for a bbq and get to know them better before asking is up to you, hopefully if it is just stored there they will understand your concern and move it. If they refuse to move it then you should be able to take action under the human rights act and report to police for invasion of privacy.
If the police don't even have a right to see pictures or videos I have taken from a public place or my property then what right would there be to see their video stream?
Go ask at the police station due to my problem i had for 7 years at my old house I was told 75% of my property and no less. This same statement was given to a pedo just over the road from me as he had a camera to protect him.
Like others that was on the same street these was told the same thing. This comes under breaching the privacy and can end up having your camera removed.
I would look more into your laws before saying so or go and ask for the advice as I had to abide by this rule for 7 years.
This camera problem is only affected in some places but in my area on the west midlands it had to be 75% and above.
Back to topic I would just knock on the door and ask if it is a camera and if so can they please make sure its not pointing to your property as you can clearly see if pointing into your garden.