I am a coded welder but in Northampton I am a bit far away.
I have seen other coded welders make a real mess of welding cars because its not the same as welding nice new metal on a work bench.
The most important things to remember when welding on a car is cut all the rust out and get the surfaces as clean as possible.
Make sure the heat is not going to damage or set fire to other parts of the car and don't do big runs of weld in one go because the heat will cause it to bend and twist.
When you are having a go on scrap metal, first have a look to make sure you are getting weld penetration. so many people think just because the weld has stuck the parts together, its a strong weld! MIG welding can give a nice looking weld that is not really that strong if not done correctly. this is not such a problem on 1.6-2mm stuff you find on cars.
Also remember to disconnet the battery.
Hope this helps