Kelly,
I was told at my 20 week scan that I too have a band...I have asked lots of questions and basically found out the following.
Bands are very, very rare and often turn out to be a fold in the sac and not a band.
Bands are only a problems they become attached to the baby,normally they attach themselves before 16 weeks and could cause problems with baby losing limbs etc. however if it hasn't attached by now, mine hasn't, then there likely isn't to be any problems.
As the baby grows it tends to kick away the band.
The chances are by your 24 week scan (they should refer you for one) the band will disappear or in fact be more easy to diagnose as a fold.
I have to have scans at 28 weeks as the band is still there now @24 weeks and then fortnightly until I have my c section.
Did they say you would be more likely to have a section?
Please let me know how you get on, I will hold your hand through this stressful time x