Bubby's heart is formed in the 5th week of fetal development. I've seen them reported on scans as early as 5+ weeks, but when you can't see one that early, do you wonder is it way too early or is it not there? So most doctors would ask you do it after 6+ weeks, if it's not there, it's most likely not there.

Doppler can usually be heard once the uterus protrudes beyond the pelvic bone (at 12 weeks), you can usually hear it around 11 weeks, I found mine 2 days short of 11 weeks. You need some practice to find it when it's that small, if you can't it's likely that you've just missed the correct spot. If you do buy a doppler, I suggest to take it in to the clinic or your GP and ask them to show you how to do it.