Is it possible to displace cerium from cerium sulphate using aluminium to get pure cerium metal? I'm unable to find cerium any reactivity series, hence the question..
No
The std. electrode potential for Ce3+/Ce is -2.33V, the std. electrode potential for Al3+/Al is -1.66V. This shows that Al is not reactive enough to reduce Ce(III), it is reactive enough to take Ce(IV) to Ce(III) so I think you would get a mixed salt of Ce/Al sulfate.
This reference here states that calcium metal is used with cerium fluoride as a preparative method.
Note: you can't do this in aqueous solution. Your reactive metal will react with the water preferentially.