Very interesting, Tam.
I have, for the last 6 years or so, used and electric propagator which has a thermostatically controlled heating element (soil warming cable) sunk in several inches of sand. This is placed below some lights (normal energy saving lights, nothing special), put 2 pots well soaked in one clear plastic bag tied tightly.
I sowed 48 lots on the 3rd March, and by today 38 have germinated, some with lots of seedlings, others with only one or two so far. The first ones came through after 6 days, 28 in all by 7 days, and the rest over this last week. I was trying some new seed sellers this year, as well as some from my established providers. Of the established sellers (Mesa Garden, Mamm Soc seed, etc.) I have 100% germination. Of the newer sellers, it is rather less successful as yet, so I don't yet know whether it is my method or the seed. It's easy to blame the seed, of course!
Interestingly, I had some seed left over of M. magnimamma which I collected on my 2005 trip to habitat, and which I'd sown that year. This has germinated again very well, so old seed if kept properly does seem OK. But equally well, some seed of M. grahamii from my trip to New Mexico in 2007, kept exactly as the older seed, has not yet sprouted. As seeds from other genera that I collected on that trip, and sown last year, have done OK I don't know what to conclude from this