This was talked about in one of the developer interviews or the regular Friday feedback pre-release. They said that radars give valuable intel but they can also make mistakes.
Also, from what i remember there was two types of radar guiding the RAF. Chain Home was the high-altitude/long-range radar that detected targets as far as some miles inland of the French coast, as long as they weren't flying too low.
There was also Chain Home Low, which was the short-range/low altitude radar. These could be static or mobile radars and had shorter range, but they could detect targets at an altitude of 500 feet (or roughly 150m) at a useful distance. It seems that if you placed some around Dover you could pick up low flying targets almost half-way across the channel, take a look here:
http://upload.wikimedia.org/wikipedi...e_coverage.jpg