I've bought a beef joint for roasting but it doesn't have much fat around it.
How can I stop it drying out?
Depending on your cooking method you shouldn't have any problem with the joint drying out. By it's very nature, a joint contains a lot of connective tissue. That connective tissue will gelatinize under a slow roast and make the meat come out extremely juicy. The best way to do this is going to be a lot of low dry heat (like the kind you get in your oven thats set to about 200-250*F). You could also slow cook it, as in a crock pot, but only if you don't want slices. BTW, if you want a crust on that bad boy, just kick your oven up to five hundred when the roast gets to about ten degrees from where you want it, and it should crust up nicely.
Finally, don't forget that all large pieces of meat are going to keep cooking even after you remove them from the heat so make sure that you take that into consideration when deciding what temp to yank it out at.
I always put meat, which I want to be juicy and tender, in a large glass pan with a lid, and slow roast it at a lower temperature. You can throw in any number of vegetables around it - sliced potatoes and carrots are my favourites, then pour in some marinade - beer, soya sauce, spices, what have you - and cook it for 2-3 hours (sometimes more). The lid keeps the moisture in and the longer you cook the meat, the tender it gets. Even game meat becomes very tender, no need to slice it at all - it comes apart when poked with a fork.
As sarge_smith pointed out, heat up the oven at the end, remove the lid and you get a thicker sauce and a fine crust.
Don't forget to let your meat warm up to room temperature before you cook it. Imagine it's just like a tense muscle when cold. Then simply brown it, about 30s each side and I always opt for slow cooking, like 130°C for >3 hours.
If it's a lean cut, such as silverside, I wrap it in foil and add about 1cm of water in the containing pan, with the lid on.