@neilw I'd put something over the top of the preserver. I have tried a few products and settled on this stuff: https://www.toolstation.com/wood-protective-treatment-preserver-5l/p48655
Its like a thin oil based preserver and soaks into the wood really well. After a few coats it leavs a slight sheen and a surface layer of oil that repels water. The colour lasts ages as well, unlike some other brands I have tried.
It does say its a preserver, but on the tin it suggests using a clear (wax free) wood preserver first, before applying this as a top coat.
I wasn't talking about your base!I wouldn't say that was way overboard, it's not like its a steel reinforced full on concrete slab. I have quite a few heavy tools in there like a table saw, pillar drill mitre saw etc, so wanted something a bit more substantial than bricks. Total cost for the concrete and timber was £112, which i didn't think was too bad for a nice solid base. It's no different than concreting in posts for a timber deck.
If the shed is just for a few garden tools, then fair enough.