Working with local planning expert Martin Small, the CPRE Mid Sussex Chair Justin French-Brooks brought the matter to Mid Sussex District Council's attention, pointing out that the advertising breached the guidelines established in the Mid Sussex Local Plan. The Council agreed, stating that the prominence of the advertisements had "an appreciable detrimental impact on the rural appearance of the locality."
The Council also concluded that the quantity of signs was an issue, citing the "unnecessary proliferation of signage." A small victory, perhaps, but it demonstrates both how effective interventions by local members can be, as well as how important it is to engage with Local Plans and the National Policy Planning Framework.
For an example of how successful objections can be made in reference to your Local Plans, download the attached letter from CPRE Chair Justin French-Brooks to the Planning Department of Mid Sussex District Council.