Have you tried to place your fondant in the microwave for a few seconds at a time (20sec)? this will soften mmf, remove from micro and knead it. If it is to soft just add a sprinkle of powder sugar then knead the fondant try not to add to much powder sugar or it will dry it out you want it nice and smooth. Once it is as your desire constancy and applicable place it on the cake. I have also at times removed the fondant from cake with the butter-cream and if there was some bc on the fondant and not crumbs i kneaded it back to its place cut/removed the pieces with crumbs and tossed away.
try this recipe it works great if you follow her recipe. I also add more powder sugar if needed you can make this weeks ahead of time if you have proper storage container I wrap it in saran wrap/kplastic wrap nto sure what you have in UK before u store it place it in zip lock then container. Once you are ready to used it just place it in the microwave for 20 to 30 at a time until it just a little warm so your able to knead it be careful not to over heat as the marshmallow will melt. If that happens just add more powder sugar to it or let it cool down. This recipe taste delicious it is my go to. I also make fondant pieces with this recipe just add the cmc tylose powder/gumpaste You can find it on line @ ( designmeacake.com) . Below is a link with Liz cakes made with her fondant watch her video its great!
PS. I have used other fondant like satin ice but had to add more sugar as satin ice was to soft with this recipe. It might work well with your store bought fondant since it is on the dry side.
Don't give up, I know its frustrating and I have been there many times I to know what you are feeling. Good luck...happy baking...