Agree with others - no marinading required. The secret is allowing the meat (especially with a large, roasting cut) to rest.
If you have the ingredients, I would make a mix of mustard, herbs (such as a mix of rosemary, thyme etc chopped fresh or dried), crushed garlic, salt and freshly ground pepper. Smear this all over the joint just before cooking and, as PP have said, plonk it onto your roasting tin atop some carrots/onion to allow the hot air to circulate. If you can't be bothered with the full paste, just season the beef.
Your top rump joint probably has very little to no fat within, so is best initially blasted at a high temperature for a shorter length of time (say 220deg for 15-20mins) and then reducing the temp to 170deg for 30-40mins, and then give that equivalent time (30-40mins) to rest, so that the meat can relax and juices can reabsorb.
Remember that your hot gravy will 'heat' the meat back up, so you can afford to serve the well-rested joint at room temperature and rare/medium, to give a juicier result.
Have a lovely Sunday lunch! Am sure it will be delicious 