Hello I am a pond virgin
I have chosen a pre-formed pond and have dug the hole ready for it, i filled with soft sand and got the pond in place all lovely and level.
I covered with tarpaulin to stop the rain getting in, unfortunately due to the high level of rain we have had the watet got under the pond and washed away all the sand. As we have clay soil the water just sat on the bottom of hole i now have a very horrible sludgy hole, has anyone any advice as to what i can do to stop the water building up under pond, i am reluctant at the moment to just put pond in backfill with soft sand and fill right up and hopefully the water underneath pond will slowly disperse away.

You have got very serious and unique problem. But still you can make it. Try to fill that area of your pond by cement or something like that. Now the question is how. So have a still road and put in the hole you have then fill it up with cement and your work is over.

Please take my advice with a bit of caution as I am also a 'pond virgin'. However I have been getting quotes to fit my preformed pond and my installer talked about a mixt of sand and concrete to keep the pond in place. I also know that you can just put this mixtture in the ground as a powder - i.e. without having to mix with water, just as you would with sand, the natural ground water then dampens the mixture and it sets.
Bear in mind I have never done this., but it might be an idea for oyu