Adult ADD is very difficult to diagnose and many family physicians do not screen for symptoms of this disorder. Also as an adult the symptoms of ADD often overlap with other disorders (bi-polar is a big one) and it often gets misdiagnosed. And with adults, ADD is often accompanied by other comorbidities, which the symptoms sometimes show up as more prevalant than the ADD symptoms and the doc misses the diagnoses again.
I hope you have a good GP, one who is not "old-school", you're better off seeig a younger doctor who is more proactive in terms of screening and diagnosing.
On this website is a great self screening test. It is used in doctors offices as well as a start to rule out or rule in ADD/ADHD as a possibility.
http://www.addcoach4u.com/adultaddtest.html There is a PDF link on this website for the test. Take the test and take in to your doctor. Remember a lot of doctors are suspiscious of adults "self-diagnosing" ADD because they think they are just trying to get their hands on Ritalin, a stimulant. A lot of addicts do in fact do this, they try to "trick" docs into getting them Ritalin by faking the symptoms. But now that Straterra has been around, there is at least a non-stimulant option.