30cm high from which point within the garden?...the lowest point or from the point of the fence??? How high is the fence 3ft, 4ft, 5ft....can you tell from the pictures???
also looks like from the new pictures, that decking was already in place when the property was purchased and higher than the new decking........
You cant just raise your garden 2ft and then stick a deck ontop and take that as the new reference. If I was the neighbour I would definitely be raising the issue with the council.