That's about a 1m gap, wide enough for a range of long, narrow tanks, and despite it looking like a really bad position it's actually the least awful place to put it. Apart from the potential issue with removing a lot of the soil near the house piles, which an engineer is looking into, the other big thing is how to connect it up.
Option 1: The outflow from the tank is connected to the garden water pipes via a tee connector with a backflow preventer (for the mains side) and we run either mains water or tank water depending on what's available.
Option 2: Mains water is fed into the tank (again via a backflow preventer) and the outflow goes to the garden water pipes.
With option 1, we can use the mains water pressure most of the time until we have to switch to tank water, but there'll have to be some switchover mechanism downstream from the tank to select which input we use.
With option 2 we've only got one lot of plumbing downstream of the tank, but we'll need to run a pump every time we water.
At the moment I can't see any strong thing pushing for either 1 or 2, does anyone have any thoughts?