I felt sick for the first 12 weeks and lost a stone in weight and about two inches round my waist. Should I be concerned?

Not necessarily. Most women are nauseated in the first trimester. If you have lost more than 10% of your pre-pregnancy weight or are severely dehydrated you may need to be hospitalized for IV fluids and nausea medications. Losing 1 stone (14 lbs) is ok if you weighed 140 lbs or more at the start of your pregnancy as long as you can keep some fluids down.
Probably not. While nutrition is important in early pregnancy, the developing fetus can usually get the nutrients it needs from the mother's body. It is not unusual to lose weight during the first trimester and the babies generally turn out quite healthy. But it is important to gain weight as the pregnancy progresses, so if you are still feeling sick please get treatment. Prenatal vitamins are also helpful.