Just googled about food being served on slates etc and it's obviously been going on for years, and yet I was unaware of it. Must get out more! Sounds horrific though. Yuck!
Re the other hygiene concerns, plastic chopping boards give me the creeps. I use wooden ones all the time – a small one for chopping and a larger one mainly as a stand for hot dishes that I take out of the oven. But every few days I put the smaller board in the dishwasher, together with any wooden spoons I've used and sometimes even my wooden salad bowl. Probably not at all necessary hygiene-wise, but works fine.