after driving at 60 mph+ on a long run my engine management light comes on and Restricted performance message for a few seconds before the Restricted performance goes off and car runs fine. after a few short runs around town the EML light goes off and continues to run fine until the next long run when the same pattern emerges, any ideas please?

 

A great place to start is an OBD reader that you can diagnose yourself. Many have used the ELM bluetooth units with an App called Torque, but there are now many more options, the car should have stored these events as numbers that you can access, post them up in this thread for more help.