Hi. I've ganache do an 8 inch cake using White choc ganache,acrylic circles and the upside down method. It looked absolutely fab until I tried to take the top circle off. The ganache came with it. 

I used a hot palette knife ran it round the edge  but to no avail  

advice pls

That is annoying when that happens, I feel your pain. Remember to keep knife against the acrylic plate when cutting through. Once you get resistance carefully take out, reheat and try again. Only pull off the circle once you have completely cut through the entire circle. Another suggestion I saw on youtube was to spray a bit of water on the circle to seize the chocolate making it easier to take off and other person covered in press and seal wrap which came off super easy but you have to spend time to get a smooth circle.

I can vouch for the Press and Seal method that Bobeba mentioned.  You still need to be careful when pulling it away from the Ganache, but it sure is a life saver.  Also be sure that the sticky side of the Press and Seal is face down against the Ganache.  I wrap it up and over the top of the acrylic circle very tightly and then use scotch tape to keep it in place.

Use baking paper on the circle that will end up on the 'top' of the cake, not needed for the bottom an most of that is taken up by the cake circle. If you put baking paper on both top and bottom you will not be able to centre both top and bottom acrylic circles due to the sliding. Just make sure to centre the cake on the 'top' plate (which will be underneath the cake for the upside down method which i prefer) then place the 'bottom' circle on and match them up perfectly. For now though you will just have to patch the cake up by reapplying and smoothing the ganache. I learnt my lesson on the first cake that I entered in a cake show - very stressfull lol. To get my baking paper right i use a stanley knife to trace firmly around my acrylic circle onto the baking paper then smear the smallest amount of buttercream (usually the SMBC that i filled the cake with) on the outside 1/2 inch of the circle and a dot in the middle then press the baking paper firmly onto the circle. Put the Ganache onto the circle then put the chilled cake onto the circle, centre the other acrylic circle to the bottom one and apply Ganache all around the 'bottom' circle (which is on top) to hold it in place, Check and fix it if it has moved then continue ganaching the rest of the cake. When done pop in the fridge and once the ganache has set enough the Baking paper will pop off by itself or with only a little lifting and you can easily slide a thin warm knife etc under the other one to seperate it as it is only stuck to a small amount of ganache :-) Hope that helps!

Hi. I followed an online tutorial that said put a grease proof circle smaller than the acrylic board then when it's set run a small palette knife dipped in hot water round edge and it just lifts off. it didn't the grease proof circle was stuck to the acrylic  board and a big chunk of cake came with it   So frustrating

That's why I only put the smallest smear around the edge of the acrylic and make my paper the same size as the circle not smaller, if it does stick for any reason I can easily seperate the baking paper from the acrylic without wrecking the cake. Also if the cake is not set firm enough the circle will be a lot harder to remove. I tried the 'spray board with water to make chocolate sieze' method on the cake that stuck really badly and I literally screamed bad words and cried while trying to get it off lol - baking paper every time for me now.