Will cooking a beef topside joint on a very low heat overnight make it dry?
If you add some liquid to the pan as well as covering it tightly, it should be fine. I'd suggest browning the joint well in a frying pan, slicing plenty of onions, resting the joint on them in the roasting tray, and then adding an inch or so of water to the pan before covering. This should keep the joint moist. With a bit of luck you'll also get some nice confited onions out of the arrangement as well.