OK -
The bear says that the key to really succulent beef is to cook it very gently... beef can get very dry if it is raised above 70 degrees, so what you're aiming to do is gently bring the temperature of the whole joint up to 70 degrees. The mistake most people make is to try to cook it too quickly in the oven. You can cook it in your frypan if you have a ceramic insert or some other thing to prevent the heat from underneath burning the bottom of the beef.
He says: get the beef out of the fridge a couple of hours before you want to cook it. This will bring it from 4 degrees in the fridge to room temp. then place it in your frypan or the oven at around 150 degrees - no hotter. If you let me know how big the piece of meat is then I can give an accurate cooking time.
Once it is cooked it is very important to let the beef rest so the juices are reabsorbed back into the flesh.
For the potatoes the trick is peel and boil as per LMS's suggestion. Once you have drained the water out of the pot, put the lid on and shake the pot while holding the lid on. This will roughen them up a bit. Then lay them on a tray and coat with olive oil salt and pepper, and lay a couple of sprigs of fresh rosemary on top. Cook in the oven until crispy. The potatoes will need a hotter oven than the beef, so if you are doing the beef in the frying pan run the oven at 180-190 degrees.
Hope that helps
And just a note from me sweet, I read your other post today and it sounds like you have been having a really hard time. Make sure you take it easy and give yourself a bit of TLC.
Hope you have a great day xxxxx