Bummer!!! did you have any scans early on and if so when did they date the EDD? The earlier the scan, the more accurate the date. I wouldnt worry about the size it can be out by 25%!!! I had a scan at 36 wks that estimated the weight at 3.5kg and there is no way I am carrying a huge baby.
Can you get in touch with the OB again? and just explain that according to your dates you would be 42+5 which I think most OBs wouldnt be keen on. They might be able to explain it better to you.
It would also be worth keeping a record of babies movements to make sure everything is going well. And keep trying your back up MW.
If you are going insane, come and join the Due in Jan 2009 thread...we are all going nuts and some of us arent even overdue yet.