Vermiculite teds to be used for tiny seeds that require some light to germinate but it stops surface drying out. Grit is better for hardy plants and bigger seeds that get get through a grit layer. Trees, shrubs and some hardy perennials like grit as it also helps break seed coats down on the above sown over autumn. Not sure about chick grit, it may not necessarily contain fugicides like i believe vermiculite.perlite does.

Hi
Vermiculite for seeds you can even use without potting compost I would also look at perlite as well these I have used with great success have a look at the link below for some good info on both
http://www.ehow.com/about_5387409_vermiculite-vs-perlite.html
Steve

For vegetable seeds I would use a thin layer of fine compost. For most perennial plants I use grit as the seeds might take a year or two to germinate and the grit reduces the amount of moss that can grow on top of the pot. A very few seeds require a constantly moist surface to grow on. This can be achieved by a layer of vermiculite on top of the compost and then the seeds sown on top of the vermiculite.

Thanks for the answers so far.
I am still not sure that horticultural grit is the right grade for seeds, it seems too large to me, all right for mixing into the compost to open it up and improve drainage but not for covering seeds. The finer grit I have discovered seems more suitable. I am running some experiments by sowing the same seed in 3 different ways and will keep a record of the results.
Taking Nickyt08's point about fungicide, I am damping the surface of the compost with a dilute solution of Cheshunt Compound before sowing. Is that something others also do?

I don't use a fungicide before sowing seeds but it is something that I could do. Most of my seed has a longish germination period and the effect of the fungicide may have worn off by then? I do use it when transplanting seedlings etc.
The grit I use on my seed pans is 6mm, the same as in the compost, as it is the smallest that I can get in bulk quantities. I have had no problems with this.

I noticed on Carol Klein's programme that she uses horticultural grit after she's sown her seeds. I normally use either a thin layer of compost if the seeds need to be covered, or vermiculite - or nothing!

I tend to put perlite into the potting mixture. If they're slow germinating (ie Tacca, Lapergaria) I'll top it off with vermiculite to stop the top going green.
With very small seeds, I top it off with a quick layer of potting compost which I've run through a kitchen sieve.

Thanks again for the added contributions. In some ways, it was Carol Klein's programme which prompted me to ask the question. She seems to put grit on everything!
Just another small point to add to my knowledge please, do others use a John Innes seed compost for hardy perennial seeds which germinate outdoors or in a cold frame? The reason I ask is that it would seem to make sense to me to use grit to cover seeds germinating outdoors and vermiculite for those in the greenhouse or propagator which are more likely to be in a peat or coir based compost.

Superscouse . . . that's what I do . . . grit for the ones in cold frame, and vermiculite for the greenhouse. I plan to mix vermiculite into the compost too this year (I'm told it holds moisture better that way?).

My preferred method is perlite mixed in the compost ( multi-purpose ) and cover with vermiculite.