In time many cars gearlevers fail to self center, the gearbox itself contains the self centerins spring
generally one of two things causes this fault.
Sticking gearchange cables, of which replacement is the only real option
A seizing or stiff cross gate crank bush/pivot
The pics and instructions below detail a maintainance procedure
that rectifies the latter.
The photographs are from a 2000W Elise, Earlier cars may differ in the bush deign, but the problem is lileky to be the same.

A - The bush/pivot that seizes preventing the gearbox return spring from centering the gear lever
B - Cross gate crank link rod, you must remove this (2x10mm spanners) to allow dissassembly of the bush.
Procedure : Unbolt the link rod at point B.
Unlock the crank pivot bolt locking nut (13mm - nearside of car end of A).
Remove the Pivot/Bush retaining bolt (next to locking nut - 13mm).
At this point the whole crank should be removed in the direction shown below by waggling it out :-

Remove the bushes (IF FITTED - IIRC not all cars have them - some are metal on metal - use your imagination)
Clean everything and apply a high temp anti-seize lube (moly grease or copperslip would be fine) and re-assemble.
It is IMPORTANT when reassembling to ensure that the retaining bolt and locknut are NOT applied too tightly as this will cause the pivot to bind.
I would advise assembling and testing before finally putting the tray back on.

Ziplockk / Driving / Lubing your Cross Gate Crank Bush
(c) Fergus Duncan 2001, 2002 [ ]