Will cooking a beef topside joint on a very low heat overnight make it dry? I bought a 1.8 kilogram joint of beef topside (called round in the US, I believe) and I am kicking myself for not buying a rib joint because it had more fat on it.  

Anyway, there is so little fat on the topside that I'm concerned it will dry out during cooking.  I've been reading up on various ways to cook it to ensure it'll be tender and not dry and I think I've established that the best way to do this is to cook it under a tent of foil for between 6 - 12 hours on a very low oven heat, ie; 100ºC/200ºF, without opening the oven door for at least six hours.  

I do have some basting fat so I'm going to put that on the beef before it goes into the oven, but I wondered if anyone could give me some reassurance that what I'm planning on doing is likely to turn out ok!  

I appreciate that nothing is guaranteed but I'd be really grateful for some advice.

Thanks in advance.
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.