Definitely, especially if your third year is weighted more as it will count more towards your grade & a dissertation is usually double weighted so again would pull up your grade. At my university we also drop our lowest module mark, so your third year work will really make a difference. You said you got 51% which is already a 2:2! You most certaintly can get a 2:1 don't feel disheartened! If you are really worried I would recommend talking to your tutor who can explain how the degree classificiations work to offer you some reassurance, and offer any support if you're struggling to get the grades you want.