Do you know what depth of sand was put down, level an area stamp it down , then dig down to the membrane, you should have no more 125mm (5") , if you have more than this your options would be remove excess sand and possibly add a 25mm rubber chip to stabilise , obviously both options will cost...
Fibre silica sand can be upto £38 per ton delivered and you would need 140tonnes
Rubber chip is around the £2k mark and you would need 20tonnes
But the sub base and drainage are really important