As you know, the problem is that the most secure wheel clamps are a bit heavy to be dragging around the county side.
Here's a link to a thread with lots of different opinions for you to be going on with.
Wheel Clamp
After 5 years ours is now showing its age but we've always made a point of covering it with one of the Protec wheel covers supplied with and as part of our van cover. By doing so, it has kept the weather off it and has stopped any dust or sand clogging up the lock mechanism over the years.
.Randa