Hello, I am solicitor with 20 years experience. I will try to answer this for you.
Are the cars parked on a public road not in contravention of any parking regs?
It doesn't sound as if he is breaking any rules regarding use of the road.
I am afraid that there is probably very little that you can do about this.
I the cars are not advertised then no the Act you refer to is not triggered.
see section below
3Exposing vehicles for sale on a road
(1)A person is guilty of an offence if at any time—
(a)he leaves two or more motor vehicles parked within 500 metres of each other on a road or roads where they are exposed or advertised for sale, or
(b)he causes two or more motor vehicles to be so left.
(2)A person is not to be convicted of an offence under subsection (1) if he proves to the satisfaction of the court that he was not acting for the purposes of a business of selling motor vehicles.
(3)A person guilty of an offence under subsection (1) is liable on summary conviction to a fine not exceeding level 4 on the standard scale.
(4)In this section—
I am afraid in what you describe there is no offence and no action you can take.
“motor vehicle” has the same meaning as in the Refuse Disposal (Amenity) Act 1978 (c. 3);
“road” has the same meaning as in the Road Traffic Regulation Act 1984 (c. 27).
How do you know\ How can you prove it?
Probably not |I am afraid. It will be impossible to do anything without creating tension but I do not think there is anything that can be done.