Your right - the quarting is out.
Turn the loco over (use the polystyrene top piece if you have it).
Unscrew 6 screws holding on the base plate. You will probably have to undo the lscrew linking the two chassis units as well.
Now, very carefully, lift off the base plate with out moving any of the wheels - you dont want them all out of position.
Lift the centre driving axle just clear of the gear and rotate one cog and lower into position - have a careful look along the connecting rods and see if they look straight. Replace the cover plate and screws and test. If not all OK, try turning the centre wheels the other way. If this dont do it - put it back as it originally was and get help. The guy from Bolton might be useful.