There's various diff models/ types, like mine you don't have to suck (to get a mouthful fish water) you shake it under the surface and sucks for ya....here's what these vacuums look like Python Gravel Tubes, Maintenance/Cleaning | Pet Solutions
Any/ All pet/fish stores you have there in the UK will carry these, the price can be anywhere from $5-50, something simple will do the trick thou.
Then you attach it to your hose and kinda run it either only over the gravel or slightly push it in the gravel, you'll see it sucking up all the brown stuff and clean your gravel that way.
Hope that makes sense now lol...otherwise I can post pictures tomorrow!?