I appreciate that I was advised to use a liner, guess I have to learn the hard way, but I do appreciate all the advice.
I dug the hole enough to fit the liner in, but now read that it should be about 5cm wider so that there's enough space to backfill, so more digging needed. But I'm now worried about supporting the shelves in the liner. I left areas un-dug to support them, but the soil has lots of stones and crumbles easily, so this area has collapsed somewhat. So this means there wont be anything solid to support them, so worried the weight of the water will push down on these areas. not sure if I can build them backup as they've collapsed quite a bit - see pic of shelf in top right, and the pillar in the hole which has collapsed.
Do I need to try and build it back up again or is it not that important ?
now you see what you need to learn the hard way. It's super easy peasy to just get a preform and voila! you have a pond. It's alluring to say the least. But imagine you have a nice preform pond, have some interesting plants and even put in fish. Now imagine you've even NAMED the fish, that your kids (if you have any, or maybe a significant other) has fallen in love with JUST one--their favorite.
Fast forward to spring and you've not paid much attention to the pond because well, ice and snow prevented you. Imagine the moment you DO pay attention and see how the water is dangerously low (or worse; it's barely got any water in it)...do you even THINK to say anything to anyone that Goldy's gone belly up? That all your hard work will now have to be redone? I have a friend who went through this and for a few years after, I saw his preform standing like some sort of grave marker at the side of his house, unused, cracked, need I say forlorn?
So the idea here is; RETURN the preform if you can and just get a liner. This question of yours wouldn't even be forum-fodder; we'd be telling you stuff like 'how to hide your liner edge' and we'd all point you to some super duper artsy fartsy drawing addy's done, showing you exactly how! And, that's where the fun just begins!
okay, I don't really even WANT to answer yoru question but I know MOMMA addy is watching, so I'll do my part; if you dig a bit larger than your preform and disregard how accurate your shelf supports are, you can line/backfill everything with sand, which WILL form up to whatever void(s) you have in/around/under/etc your preform, shelf or otherwise.
Okay, done. Did I avoid the corner and a timeout, MOMMA addy??