The following applies for under 5ha.
The Town and Country Planning (General Permitted Development) Order 1995
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Class B Development on units of less than 5 hectares
B. Permitted development
The carrying out on agricultural land comprised in an agricultural unit of not less than 0.4 but less than 5 hectares in area of development consisting of—
(a)the extension or alteration of an agricultural building;
(b)the installation of additional or replacement plant or machinery;
(c)the provision, rearrangement or replacement of a sewer, main, pipe, cable or other apparatus;
(d)the provision, rearrangement or replacement of a private way;
(e)the provision of a hard surface;
(f)the deposit of waste; or
(g)the carrying out of any of the following operations in connection with fish farming, namely, repairing ponds and raceways; the installation of grading machinery, aeration equipment or flow meters and any associated channel; the dredging of ponds; and the replacement of tanks and nets,
where the development is reasonably necessary for the purposes of agriculture within the unit.
So from reading this surely you can make the repair under TTCP Act Schedule 2 part 6 para B. as you are making an alteration to an existing agricultural building in that repairing the building is fine. as from what you are saying the walls are fine other than a lean. So you are going to repair the roof and floor and to do so you will need to remove the walls as lomg as you do use the same walls again I cannnot see any reason that you cannot use this section.
Tala