We all have close shaves, most do not get reported, some are closer than others. In VFR-type flying there are some measures you can take to reduce the risk of a mid-air collision. Know the high risk areas - especially when joining a circuit. Always look and listen for other aircraft nearby wherever you are. Use the Radio and whatever means you have available to visually highlight yourself to others, colour, lights, transponder.
There were 2 GA fatal mid air collisions in the UK in 2011, both occurred in the vicinity of airfields and both involved aircraft rejoining the circuit, links below.
Other good reading material in the links below including an excellent GAPAN article that, while dealing primarily with sense and avoid safety levels for remotely piloted aircraft, has huge relevance for GA fliers.
Finally, a link to an EASA website on How to avoid airspace infringement & reduce the risk of mid-air collision.