Thanks ladies. I do feel a bit more reassured but I think I probably feel less so than yesterday! I don't know why. The dating was a bit weird. The first doctor (who, coincidentally, was an old school friend) asked me how far along I thought I was, I said 10 weeks, she scanned me and said "Ten weeks exactly" and said the baby was measuring 3.4cm. So that was fine and I was really pleased. She had a bit of bother with the settings on the scanner and measured twice as it came up with two different measurements, but again, at the end, she re-confirmed the 10 weeks.
Then I had the rest of all the appointment bits and then had to go and see a different doctor again at the end (I think because I had a third degree tear the last time so they wanted to check if I wanted to go for a vaginal delivery this time or not - I do.) The doctor looked through my notes and said, okay, so we're 9+1 and I said, erm, hang on, the other doctor said I was ten weeks exactly. She said oh, a week out is totally normal at this early stage. But I wasn't happy about it and asked again later in the conversation, and she said, why don't I scan you again and then we can see. So we did that and she said I was 9+4/9+5, which is still a slightly less, but made much more sense.
When I came home and looked at my notes, the 9+1 bit had a measurement of 3.0 something cm, so I think what might have happened is the first doctor noted down the wrong measurement from when the settings were a bit funny. And THEN, when I looked at my notes for my due date they had written 19th August which is 2 days ahead of what I thought in the first place....aaarrggghhh!! Obviously, saw the heartbeat which was great and I know there's such tiny margins at this stage, but still - a bit annoying!
What do you guys think? 
Sorry for long post!