How can I accurately diagnose fault in Peugeot 206?

Peugeot 206, 1.4 litres, 03 Reg, 50400 miles, Petrol
* Engine diagnostics warning light has come on (when driving at about 60mph on the motorway – last Monday 7th December) and remained on since (there was a slight juddering at the time but it has driven fine since). When this happened previously, again on the motorway, I called the AA who reset the light and couldn’t find a recorded fault code on the car’s computer. Aircon switd doesn't always work either.
* Indicators work intermittently, especially when the headlights are on (happened a few times since I’ve had the car, although was at its worst yesterday where it turned off every time they were used).
* Horn started when car was unlocked and didn’t stop until I’d pressed the horn again (on Thursday).
* The warning noise for headlights left on when the car is turned off came on yesterday, although I knew that the lights had been turned off (yesterday).