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 [