There are a few simple and effective ways to ensure the polythene is in place correctly.
Firstly, check that the sand blinding covers the whole foundation prior to laying the polythene. This layer prevents large rocks from creeping to the surface and puncturing the membrane while people are moving across the platform.
If possible, lay the polythene before pegging the bracing into the ground. This will allow you to pull the polythene beneath the upstands and past the edge of the foundation, making it easy to ensure that the polythene extends at least 100mm past the foundation as required. It also avoids bunching of the polythene between the bracing and the profiles which can effect the ability of the profiles to sit neatly against the bracing, thus ensuring a standard 5/6mm overhang is attained for the frames.
Finally, when pegging the bracing, try to place some of the pegs through the polythene (although past the 100mm requirement). This will stop the polythene from creeping towards the foundation as the concrete is poured.