bullworth,
That
is the preparatory list! LOL!
What I won't do is contact the manufacturer and get their recommended details for the base and how the shed should be fixed to it - that's all your job.
I suspect there is a step detail at the edge of the slab for weathering purposes that will need careful measuring out and attention to forming the edge with a proper edge trowel.
Even with most of work on the above list done for a driveway I put in myself, I still paid one of the concrete delivery guys to help me with spreading the mix, leveling, brush finishing and troweling the edge.
This is important when undertaking building work and particularly self-builders - thinking because you can read a post here you can build is simple wrong - ask for site help from people who know.
For my driveway, I had taken deliver of 65 Kn concrete, very strong stuff for permanently uncovered northern weathering exposure that went off in an hour so we had to work very fast.
You prolly won't need that kind of strength for a shed base, perhaps 25Kn ewould suffice, but you still have to prepare the ground properly.
Personally I wouldn't be building near trees at all.
If you raise or lower the ground level by 150mm / 6" it can affect the tree and destabilize it.
You can ring up the parks department in the local authority and ask them for advice on building sheds near trees.
Green activists will advise you to put four steel stakes in the ground and span between them - that's a newer way of thinking about the whole thing.
ONQ.
[broken link removed]
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.