Sue, you'll only get to see the HB at an early scan. I'd say you are in with a reasonable chance at 6w1d, if they can't see it externally they'll just do an internal. BTW, I had my first scan externally at 6w6d and could see the HB clearly .

I didn't actually hear bub's HB until I was about 16 weeks along, although it is possible to hear it a few weeks earlier than I did. A lot of caregivers won't try to listen with a Doppler though before about 14 weeks or so because you can't always hear it then. Good luck!