Well I have not got an early scan as such - the midwife said they'd send me on May 24th for my 12 week scan...which thinking about it now is wrong, as I'll only be 10 weeks by then! Dunno what the logic is there!
They can date you at that time, I think the range is 10-14 weeks that you can have your "12" week scan, depends on the hospital work load.