You cannot ask your employee to do any work that:
makes money for your organisation
provides services for your organisation
They can take part in volunteer work or training.
My thoughts on this (and i have NO legal standing!!) are that if your staff are volunteering their time to keep in contact that is up to them, but it needs to be clear that they are volunteering and this has nothing to do with their employment. They can't be Asked to do it. I had thought that if we were topping up the 80% that that might cover it but the government document does make it clear that this is not the case