I brought one with DS3. Yeah I have just been able to find the heartbeat myself. I tried last week but failed and worried myself silly that something was wrong. I did that last time with DS3 so should have learnt my lesson.
It is lovely being able to hear it whenever you want but in a way it causes a lot of stress for me as it can take me 15 mins to find it and in that 15 mins stress levels are through the roof.
Hope you get to hear it soon. I'm sure you will at your next appointment. It's a shame we don't live next door I could pop mine round :)