Today I spent the day walking around town trying to think of the best ways I can overcome the problems I am having with my practise. I was unable to find any double sided sticky sheets, I looked in all the art shops, including whsmith, rymans etc. as well as homebase. I ended up buying some adhesives from hobby craft including sticky dots, a peel off glue and a thin double sided tape. Although I can’t find sheets of double sided sticky plastic, I may be able to stick the stencil down in the parts where it is needed.
Alternatively, I could try and iron the plastic with a sheet of parchment paper over the top to flatten it. Although this could cause the image to become warped, I would have to make sure not to heat it too much.