Ok, morning dress is a step down from black tie. So no ball gowns, but if its a winter wedding, you may get away with a cocktail dress, so long as its understated (not too many sequins or feathers!). If it isn't summer, a smart pants suit (or indeed a skirt suit) may stop you being cold. If you've got the invitation now, I'm presuming November wedding, so its likely to be cold, so unfortunately the summery frocks which are the stand by may just be too cold.If its a summer wedding, bizarrely black is less appropriate, but in winter, difficult to get away from. Elegance is always good however.

Hats are really your call. For a society type wedding, they are de rigeur, but increasingly not worn. I always do, because I like wearing them

Ok, here's some stuff I would consider suitable (this is just to give you an idea really since I don't know anything about you!) And as I said,smart summer frocks, but they are not in season, so I can't find any photos to show you what I mean.
http://order.next.co.uk/page.asp?b=G48&p=12&o=1
http://order.next.co.uk/page.asp?b=X34&p=48&o=1
http://order.next.co.uk/page.asp?b=X34&p=50&o=1
http://order.next.co.uk/page.asp?b=G48&p=18&o=1 - although probably not the halterneck (too much flesh showing!)

Hope that helps!