winjim
Smash the cistern
I think the builder's a bit of a red herring tbh, and the drainage. Here's a picture from a different angle:It might take a while to recover from the builder. If you want it ready for the little one later this year, I would suggest that you get some quotes for turf.
Then avoid going on it when it's wet. Protect it from future builders with planks and pallets etc.
It's the patchy bits in the foreground and under the bush at back left which concern me most. My preferred course of action I think would be to reseed it, but I would like to know how, when and what with. Anything that involves major upheaval or expense simply isn't an option, so no digging up for additional drainage etc. This was done two years ago and isn't necessary I think.