Hedge Advice

Hello All,
Nice forums some good topics going on here :)
I have a boundary to my garden which is adjacent to a public footpath approx 50m long. Approx 40m of it has 6ft fencing with hawthorn hedge at about 8ft tall to the footpath side. The hawthorn runs all the way down to the bottom but is very thin to the last 30m as it has grown so tall i.e. 20ft. I have cut all this back down to about 6/8ft but still this hedge is probably in partial shade due to two large neighbouring beech trees. I need some privacy to my garden as the hawthorn is hedge can be easily viewed and passed through. I have toyed with the idea of planting leylandii behind the hawthorn just to act as a fast growing screen until the hawthorn picks up (if it does).
I love the hawthorn because the birds like it and it has excellent security values, but the leylandii sounds good as a fast growing hedge but the horror stories and the fact its not native make me nervous. How would these two interact so close to each other i.e. hawthorn and leylandii 2 ft apart? Does anyone have any thoughts on this being a good or bad idea?

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