thanks Theotherbrian, this is exactly what I was looking for.

For any old codgers like me/the visually impaired, I've adjusted the map to show the SE23 contours a little more clearly, at the expense of other details. (If you zoom in, obviously)
   

I'm not sure how well this corresponds to the geology map though.
There are some questionable very straight boundaries for the newer clay deposits, imo, so perhaps the survey is incomplete.