30 days seems long enough we have had them stall for many months sometimes you just have to draw a line in the sand. There is case law that states Allstate Vs. Ford I think if you take evidence from the scene for safe keeping it is not spoiled. I would set the date out a couple more weeks and let them know time is of the essence and that its all on them if they want to attend and if they dont they give up all rights to any spoil claim. They cant claim they were not warned or noticed but do it by certified mail and email both so they cant claim they didn't get it.