do they have planning permision for their decking because they may well need it
You have a 6ft fence, but your neighbours can look over it into your living room? The quick answer is "lace curtains" ... which my wife hates.
Hmmm! If the gardens sloped UP, they could see into your living room. If your neighbour was unduly tall, he could see into your living room. If you had the more normal 4ft fence, he could see into your living room. But I doubt that you wanted t know that.
Decking should not be more than 12" above ground level. But your neighbour is probably relying on the convention that measurement is taken from the highest point immediately next to the building. So a question for the experts ... if you can build a shed that is 2.5m high at the end facing you house, and 5m high at the end facing away from your house, provided the ridge is horizontal, why cannot you erect a fence that is 6ft high where it meets your house, and 20ft high
at the bottom of the garden, provided the top of that fence is horizontal? I've never noticed that loophole being closed, in the regulations .... but I bet it is. So, just asking!