Richard, check http://www.siteseeker.co.uk/aspx/search.aspx. It's the CCC's own site finder. I couldn't see anything for Broadstairs, but there are several listed for Sidmouth.

A word of warning though, the locations listed aren't always a reliable guide. EG., a Simouth listing doesn't necessarily mean it's anywhere near Sidmouth. It could be that the camp site propietor has given that as the name of the nearest postal town.

On the other hand, you may find a site which has been listed under the name of the nearest village, and could actually be closer to Sidmouth.

The best thing, if you can use an OS map, is to check the grid reference. They're always quoted in the CCC guide. Alternatively, key the post code into Google maps. That will show you exactly where it is.