Erm, its not really okay to put manure into the ground and then sow seeds - it's fine to plant immediately with actual plants, but seeds sometimes don't respond well. Still, if you dug it in well, some will probably germinate anyway, and if its only one bag of composted manure, that's not much. In future, add as much composted material as you can get hold of, either in autumn or early spring, and wait a week or couple of weeks before sowing or planting to allow the ground to settle after digging.

Also, wildflowers often do best on poorer soils. To open up your clay you could try adding some coarse grit as well as some compost. I would have thought that bought manure from a GC should be well enough rotted not to burn anything though. Do you know why seeds don't respond to newly manured ground even when the manure is well rotte, Bamboo? I have just manured some of mine as i wasn't able to do it in the autumn, so it would be good to know. (Sorry to but in on your question, Fuzexi)

Some might thrive on it, some don't, Steragram - depending on how well rotted the manure is, it could burn not the seeds, but the seedlings as they try to grow. Very good point about poor soil being best for wildflowers too, didn't think to mention that myself...

I won't worry then as mine is well rotted.

Thanks to all of you for your replies. I think I will water it well then leave it for a few weeks and reseed if nothing's coming up...
I originally went to the GC to get some sand, but they were out of it. They had some sandpit sand, but I didn't know if it was straight off the beaches and full of salt. Then I saw the horse-poo and it said 'good for opening up clay soils', so I went for it without thinking.
I haven't done the whole garden, so I'll put some seed on the original ground to see if there's a difference.

I did all this for 10 years, and the sub-standard clay soil never improved. Thats why I started my Container Garden. All my plants are now in 'Clover' compost, and its a real thrill to watch them thriving !

Don't waste your money on sand to loosen your clay soil, Fuzexi - as much composted material as you can afford as often as possible, and some sharp horticultural grit, all dug in, will do a much better job.

Thank you everyone for your comments, and my apologies for not replying. Got really busy and actually forgot the name of this website. Anyways, found it again, and hope to set up a gardening blog!
The wildflowers are doing really well, I've got scarlet pimpernel, borage, poached egg plant, rocket, mustard (or something that looks like mustard), dock (don't think that was in the mix!) and two species of nettle, one with purple flowers and one with white)' daisy, buttercup, little tiny blue flowers, and a lot of stuff I don't recognise. I'm going to try to upload some photos...