Didn't Carry on Behind kinda rip this off?
I watched it the other day when shown on Channel 4 and its got an almost identical plot only with different actors and both are set on Camping sites. Oh and I actually really loved "Behind" due to Kenneth Williams and Elke Sommers great chemistry together but it wasn't a patch on this one.
They think we're the demons now - Prue Halliwell ~ All Hell Breaks Loose ~ Charmed