I find Iceland to be great - lots of slots, even for next day delivery, and it's rare that anything is missing or substituted. Tesco have been good, too. I haven't had great experiences with Morrisons - last time, literally half the things I ordered were missing so I had to immediately put in for another delivery elsewhere. I haven't used them since, not impressed. I'm guessing this kind of thing varies depending on the store near you, ut maybe some chains are just better than others.