Firstly, old shed (rotten, plant-covered) needs to be removed from back garden.
Then a shed similar to this needs to be picked up from B&Q Chiswick (2 miles away) and installed in its place.
https://www.diy.com/departments/forest-garden-6x4-ft-apex-overlap-wooden-shed-with-floor/5013053176795_BQ.prd?alt=true
The existing base may not be sufficient, so concrete required before installation - advice upon quote would be helpful.
Maurice was an absolute legend, bringing my shed back to life. He came to quote for installing a new shed, but he identified that the old once could be fixed, which he did, including clearing a load of plants growing all over it. Very efficient, good communicator and brilliant workmanship.