Assured by whom in the Council?
I suspect that the Planning Dept. couch their pre-plannings with sufficient conditions that such assurances are non-binding on them. I'm not certain though so you'll have to check / ask someone who knows, such as a Planning Consultant.
If there aren't any such conditions attached to the discussions, perhaps you can argue estoppel if you go the legal route(i.e. Relying on their assurances, you submitted a particular design (if you can prove that) and that they can't go back on their promises).
Other than that, County Councils have pretty much a free hand to determine what they require to meet Part V requirements in their areas. So unless there is some way to tie them to their assurances, there is nothing you can do.