Please break apart my idea and find as many faults as you can.

[Scythe13]

Could investigate underground/covered IBC Tanks

You might also look at this commercial rainwater harvesting offering. It may be far too expensive but it may give some ideas.

I'm looking at actually getting a load of pond liner material to build a massive pond, if anything was to happen.

With the IBC's, we're hoping to get 3 (2 more to go) for water collection, maybe sneak a 4th for water, and at least 1 extra for an aquaponic system.