Inserting the wheels and gears looks easy, but is quite tricky. Place all 4 gears into their (deeper) slots. Start with the rear set of wheels (they have tyres). Remove the connecting rod from one side only - prise the pins out by twisting a screwdriver between the wheel and the coupling rod. (see photos in the Dismantle section of the Mallard). Drop the first axle into place (check the spare end is facing the middle axle position), and rotate the wheel so that the crankpin hole is at the 12 o'clcok position. Easy. Now drop the 2nd axle in. Then spend a bit of time ensuring that the hole is at 12 o'clock. As you do so, note that as you rotate the wheel, the plastic cog will rise out of its slot, and the axles will rise out of theirs. When you think you have them both correct, push the downwards on the gears to make sure that they are seated, and check the positions again.

Now with two wheels and one red cog to keep in place, repeat the operation for the middle axle.

Then with 3 axles and 4 cogs to hold, drop in the other pair of wheels (Again easier if you remove the coupling rod from one side (the same side as before!). Each time, ensure that the wheel is in its proper position before moving to the next wheel. You have to do them in sequence - rear to front or front to rear. Note that there is a little play in each axle/cog. Make sure that all of the slack is taken up in (say) the clockwise direction, before checking the alignment. And keep checking that the axles and cogs are pushed down fully into their slots.

You may wish to place the bottom plate in position after fitting 3 axles so that you can be sure that the rear 3 are definitely properly located, before moving on to the front pair of axles. Removing the bottom plate again without lifting the axles can be tricky though.

As a final check that all 5 wheels are properly postioned, turn the engine round and look at the other side. Whatever the position, the spare end of the coupling rods should be hovering over the hole in the centre wheel, and the coupling rods should form a straight line across all 5 wheels. If not, something is mis-placed ! Ensure that the alignment is not caused by some wheel being turned one way against the free play while others are turned the other way.