I have just had my 20 week scan and the nuchal fold was 6.3mm, does this mean my baby has downs?

Not necessarily. 6.3 mm is above the 6 mm cut-off but that doesn't mean the fetus has trisomy 21. At your age however, an amniocentesis would probably be suggested to determine if there is a chromosomal abnormality due to the increased risk seen in older mothers. Did the ultrasound suggest any other abnormal markers like short long bones or facial abnormalities? Good luck.
No. The nuchal fold was developed to be assessed at 11-13 weeks, not 20. Even when it is increased, it doesn't mean the baby the baby has down's, just that there is increased risk it could.