If the Acers have been growing fine till now on your patio then it must not be that exposed as they do not tolerate wind!

It is too much water that kills most Acers, so putting their pots on feet (you can make them from wood) will help drainage and see them through winter.

Unless it gets well below -5 for days then there is no need to fleece them.

I've got one in a pot which I put under a bush for the winter. It's survived no bother for about 5 years!!

thanks for your advice. They have only been on the patio this year. i do have them standing on something.i will just leave them unless it gets really seriously freezing.