I had a similar issue when I installed my 30,000l tank, the digger operator dropped a load of sand on the top thinking it would fall off and fill in the hole it was sitting in. Instead it collapsed the top....
I got in there with a 6x6 post and a scissor jack and slowly wound the jack up in a couple spots and it pressed out sweet, after a few hot days any sign of any crease etc is fully gone.