Frank - I did not say on the roof but in the roofspace/attic. The tank will have a lid but it must be removable for cleaning and repairs.
I happily brush my teeth with water from the bathroom tap, but drink water that is freshly drawn from the mains because it has not been sitting around for hours/days in a holding tank. Public washrooms that do have an "indirect" supply to the taps should post warning notices that the water is not suitable for drinking.
http://www.housewiz.co.uk/plumbing-wizard/safe-drinking-water.htm
Sorry to have caused any alarm. Having grown up with it, it is just second nature to not drink from anywhere but the kitchen tap. It was seeing the comments about filling water bottles that suddenly reminded me that visitors might not be aware of something that we take for granted.
PS ask your accommodation provider whether the water supply in the bathroom is ok to drink.