You should check with the planning office or read precisely what is outlined in the Further Information request. A number of planning authorities in recent years have sought full legal agreements and not just a "letter of consent". The formal legal agreement usually includes a map showing the extent of the lands so affected outside the site boundary together with an undertaking from the third party’s solicitor that the agreement will be entered as a burden against the title of the land.
There have been numerous cases where letters of consent (not worth the paper they're written on) have been renaged on, which has left a situation where sightlines are not provided and/or maintained, it constitutes a traffic hazard and there is little the planning authority can do, as the sightline condition is not enforceable.