Yes, you're probably right Baz' and I concede the point (the point being that it's speculation to say that long barrows go a fair ways back).

It's worth pointing out though that long barrows certainly do go a fair ways back even if the stone chambers do not, and there may have been flimsier chambers in the earlier stages of a long barrow that have since long disappeared. I think it's also worth pointing out that 'excavating' and 'examining' (first line of your last paragraph) are not the same thing and that until all unexcavated longbarrows have been excavated we can't be entirely sure what's going on.