the crackling noise will be the engine pinking. you really are better off taking it to a garage to get it done, especially if your not too confident at doing it yourself. 40 -50 £ should be about what they will ask for, and if its still pinking then you might want to check your knock sensor (located just under where your altenator is attatched to the main block) check all the wiring as this should be well insulated! if not, its an expensive little bit of kit. so if the tune up doesnt work, then get saving, around 40£ i think...
thats all i can help with im afraid...