Stan The Man wrote:
Part of my garden originally belonged to the neighbouring farmer; the
previous owner of my property bought half a field from him 15 years
ago and has since integrated it into the original garden here - laying
turf, planting shrubs, creating borders, laying a brick patio, putting
up a couple of garden sheds, etc. However, she never applied to the
local authority to change the use of the acquired land from
agricultural to garden use. I'm led to believe that it should be
straighforward for me to make an application now for garden use based
on lawful development over more than 10 years. But I am curious to
know why I need to do it. What can't I do on agricultural land that I
can do on garden land - and vice versa? TIA.

You can apply to the local planning authority for a certificate of lawful
development that is normally granted. You can only use agricultural land for
agricultural purposes not as a domestic garden.