Is it possible to displace cerium from cerium sulphate using aluminium to get pure cerium metal?
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.