Vernalizing perennial seeds

The easiest thing to do is just leave them outside and they will germinate in spring when the weather suits them 

It depends what you've sown - but some perennials take longer than 3 weeks to germinate anyway

I remember getting some primrose seeds from Barnhaven years ago, and they recommended scatter-sowing on the top of a small tray filled with compost (was it multi-purpose - think so). Then you gently water in or soak from underneath then put another small tray as a lid on top, plus a brick to stop it blowing off.  It shouldn't dry out over winter, but examine and remove lid on signs of growth in early-mid spring.

What a brilliant link, thanks for that, I have bookmarked it.  I have noted two main points from this:

1.  I am being far too impatient, 3 weeks is obviously a short period of time for perennials.
2.  Not all seeds like the same conditions, so putting them all in modules in the same seed tray will not suit all of them.

My general seed germinating technique has been to bung them in a tray or pot of general purpose compost, water, and leave on a window sill, greenhouse or heated propagator, and this has usually worked for me OK up to now, although I have only grown vegetables and annuals from seed up until now.  I obviously need to refine my technique a bit for perennials.  I will try to move them to seperate pots and give each one more favourable conditions.