Thanks for all the comments. It's great to share people's experiences.
Couple of things to reply to though.
Last night was probably the coldest ride I've done this year. Some puddles were actually frozen, so I guess it was at best 2 degrees. I had shorts on and knee pads so my legs were fairly exposed but warm. I also had SealSkinz mid socks and Vans style trainers. My feet remained warm through out the whole ride. My shoes and legs did get a good slashing on a couple of stream crossings but not to the point of saturation. I did the same ride the week before, but on a dry afternoon and by the time I'd finished, I couldn't actually feel my toes and I had the SealSkinz on then too.
So in summary I can confirm that for me, flatties and trainers are a whole lot warmer than SPDs. Although for summer I'll probably go back to the SPDs.
As for technique, I did really concentrate on weighting and un-weighting as opposed to just relying on being clipped in. Through some fast downhill berms I felt I went quicker than with SPDs as I was forcing the bike in to the turns more.
I even tried a 12 foot long tabletop and a 90 degree turn hip jump. I managed to keep both feet on the pedals and landed smoothly. In fact my foot only left the pedal once, when I'd totally forgotten about not being clipped in. I reckon that another 3 or 4 rides of intense concentration and I'll have it nailed.
I'll keep you all updated.