The first question you need to ask is whether there is in fact a 75 metre sight line there already. A sight line is often shown\demonstrated on a map, but this is one situation where there is no real substitute for physical inspection.
Simply put, your planning requires that when exiting your site (presumably onto the public road) you should be able to see 75 metres in either direction. If you can't, then changes need to be made in order to meet this requirement.
If those changes involve cutting back the neighbours hedge, or moving the neighbours wall, or reserving the right to trim their hedge in the future etc. then you need an easement from them. If, however, there is already a 75 metre sight line, and if the layout of the road and adjoining land is such that it will clearly always be 75 metres, then you do not need an easement.
These planning conditions are often imposed as a standard wording without actually checking whether they are needed. So you might have a planning condition which says that you have to 'provide a 75m sight line', but the condition might be redundant in that there is no actual work needed to either create or maintain such a sight line. So don't panic until the architect has examined the situation on site.
Your architect must certify whether or not the sight lines are attainable with your present site and entrance layout. If the sight lines are not attainable (AND maintainable) without your neighbour's co-operation, then you can't build (or rather, you can't be in compliance) without getting the easement from the neighbour.
And, as MF1 states, this should have been flagged during the planning process.