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Doggit
You don't mention what sort of difficulty you had drilling the wall, but if you had a concrete lintel above the window, then you need an SDS drill rather than a normal hammer drill. It's also possible that the rawlplug wasn't the right size for the drill, as they are not as standard as they should be. The plug needs to be a tight fit when it goes in, and preferably towards the back of the hole, rather than the front where the plaster can be soft. If you can get longer screws of the right size, (and assuming you can drill in deep enough, then you can whack two plugs into the wall, and that means you're definitely in deep enough to hold the rail.