Can you cook a beef joint in a slow cooker,if so how.

Yes you can. Put the slow cooker on high to begin with. Whilst that is heating, take a frying pan & brown all sides of the joint to seal it. Season the joint if you wish & just place it in the cooker, I don't add fluids at all. Depending on your cooker, you can leave it on high (check your manual). Or perhaps leave on low & cook over night, the gravy will be great from it & the house will smell beautiful!