Dave wrote:
Its too long for a compost heap, its too narrow for a shed, I need
somewhere to put my Petrol mower, dirty garden tools, garden rubbish
etc ?.

You could, as suggested, add a roof and a door, but it would be a long
way to the back. So add a door at each end, and make two sheds of it,
neither so deep that it becomes a glory hole.

Other possibilities:
An array of water butts, with a remote tap on the end of a long pipe.
Add the roof, and the partition half way down. Then knock through from
the garage to make a walk-in cupboard at one end, and the shed with
it's outside door at the other.
A small roof and some bars sticking out from the wall, and use it to
air-dry (season) timber for furniture making.