The area of development land for cgt purposes is complex and rather tedious.
The "only" difference between development land and non-development land is that the former does not qualify for indexation relief on its development value. Only its current use value is what is allowed for indexation. As the land was acquired prior to 1974 the valuation date (as you probably know) is April 1974. You will probably need to get a current use valuation at April 1974 and a "hope valuation" at the same date. The former valuation can be indexed for inflation but the latter cannot. Any valuer will be familar with the difference in valuation as this is a common issue.