Hi Ginley,
As others have pointed out, most houses have government permitted development rights, and under such rights it is normally possible to install decking in the rear garden up to a maximum height of 0.3m above natural ground level without requiring planning permission from the Council.
It's important to note that this height is measured from "natural" ground level. So it's NOT possible for someone to (say) put down earth to artificially raise the level of their ground and THEN install 0.3m worth decking on top of this raised level. The 0.3m is the total that you're allowed - so if they've already artificially raised the level of their ground, then they're going to have less allowance remaining to install the decking.
It's also worth noting that where the natural ground level is sloped, the government legislation allows the measurement of this 0.3m to be taken from the HIGHEST natural ground level adjoining the decking. Unfortunately, this means that with a significant slope in natural ground level, it's normally possible to install decking that is 0.3m above natural ground level at one end, and then keep the decking on a level plane meaning that it will be much higher above natural ground level at the other end (this is permissible under the legislation).
As others have said, if you believe that your neighbour has installed decking that is higher than 0.3m above natural ground level, then it's probably best to contact your Council's Planning Enforcement section, and ask them to investigate. As others have said, it's polite to give them a chance to respond by their normal procedures, rather than going straight to a Councillor.
Hope the above helps
Steve