... I dont want to be told I am measuring behind if it would be due to having a slightly longer than average cycle (I hope that makes sense)
I think that since it is specifically a dating scan, ie, a scan to determine how pg you actually are, whatever the scan says will be taken as the date. When they ask when your lmp was, say it was 'around the ...th of February' or what ever, and that you have long cycles. if they ask how many weeks along you think you are, just be honest - say that you reckon you're X weeks and days, cos you have long cycles.