Rear garden

We have decking in the back of our garden that has rain water storage tanks underneath and a well connected. We want to get rid of it and have turf but a bit stuck how to get there.

Once the decking has been lifted there is probably a half meter gap to the tanks and once these are removed they'll need filling. Can we just fill with soil or could that create a drainage problem?

Behind the well is a gate directly to the neighbours rear garden ( not our choice, she was close with the previous owner) but it slopes down. Unsure how to level without creating a run off of rain water to her garden

I know some will say keep the decking, but it needs replacing anyway as it's dipping and rotting. We don't use the well and just want to take the garden back to basics and build upon, we hate all the concrete and gravel but unsure how to start and what to do after.

Here's pictures of our garden, we want to remove gravel, level, remove concrete as possible and lay turf, but want to do it properly. What would you recommend as the garden as it is is a concrete gravel jungle


Edit- so the tanks need removing, it's not for us. The water storage tanks are length ways of the decking but I don't believe are deep and yes connect to the pump/well thing. It's collected from a makeshift guttering system and tights used to stop leaves going in.

We have a toddler so we just want lawn and we can add pretty plants as he gets older but as it stands he just wants to throw gravel. Everything has been done quite shoddy so we want to start from scratch.

Could we fill the hole under the decking with gravel and soil?

We want to do this ourselves over time, but wanted advice off others as to how to get there.

A big project! Are you going to DIY or call someone in?
One of my first things to do would be remove/seal that gate opening. It doesn't matter how well you get on with your neighbours, (and we would all like to), allowing them free access to your garden/property is a definite no-no.
When you say there is a well supplied by two rainwater tanks are you referring to those pump handles? In relation to the pump handles where are the storage containers? If they are not directly beneath them there will be some form of pipework underground. As it's a 'bespoke' system I don't think it will be very deep except for the tanks. It may be possible to remove them or simply seal them off to prevent more water collecting in them. Do you have any idea how large they are?
Access for a mini digger doesn't look like a viable option unless you have a side gate out of view.

If at all possible, keep the tanks. You can never have too much water storage, and half a metre is plenty deep enough for the soil for the lawn. I'd remove the decking, put some kind of cover over the tanks to protect them, cover that with a thin layer of concrete rubble from where you have removed it elsewhere in the concrete jungle, then topsoil for your lawn.
You don't use the well presumably because you currently have concrete and decking - once you have lawn and plants, you'll likely regret it if the well water is not available.
If there are really really good reasons not to keep the tanks, then fill in the space they were in with concrete rubble from your garden, then soil on top.

I'm a little confused how levelling a slope would cause more runoff?

Personally I wouldn't worry about the gate - whist you might not choose to put one in, it's a bit of a waste of effort to remove or seal it if it's not causing you a problem (and might be practically useful in the future, who knows?)