just wondering how early the sex of the baby can be found out, is it only over 20 weeks

I think you can find out from around 17 - 18 weeks but scans in Sheffield tend to be done at 20 weeks.
I guess with someone prepared to spend the time looking, it can be found earlier.

I have found that they will only tell you at the 20 week scan, I had a scan at 17 weeks the day after a car accident to check everything was ok and asked if they could see but they wouldnt tell me. If your willing to pay for a private scan then you could find out earlier but im not sure how early on they can see what the sex is

The external sex organs form at around 13 weeks but aren't reliably visible until around 18 weeks, hence they usually wait until the anomaly scan. At WestHerts PCT where we were they wouldn't say at all. My cousin (who has worked in imaging departments) said they often won't tell in areas where certain cultural groups are 'selective' with the sexes of their children. :( We had to pay privately to confirm what our eagle eyse spotted on our 22 week scan - Baby Alien happily flashed his bits at every opportunity, but always covered his face!