I intend to take feed from kitchen ring main through a DP switch
and a 13A fused spur unit to a 4mm.sq SWA cable running to the
shed. I intend to fit some RCD protected mains sockets in the
shed, and take a feed through a 5A fused plate for lighting.

Rather than use RCD protected sockets in the shed, I'd use an RCD fused
connection unit instead of the DP switch/fused connection unit you intend.
This will mean that the cable run and lighting is also RCD protected (making
low earth loop impedence a belt and braces approach, rather than critical)
and be cheaper if you are installing more than one socket.

Christian.