Moto - Well at least it doesn't have an A on it! So it could be a UK one :-)

I really do not now know what you have as a lens on your ST. I am not aware that any of the lenses were made in plastic - all those I have ever come across are glass. Maybe the newer ST's have a plastic headlamp lens, but I thought you had an ST2.

The lens cover does the deflecting. Both beams are biased to the nearside of the road - no beam is absolutely straight on. They are biased left or right dependent on whether you drive on the right or the left of the road.

I'm interested in what your local dealer will say.

It may well be glass: I wasn't looking at it when I said plastic.

The main beam should point straight ahead. The dip beam can have a straight cut-off (US dip) or can have a cut-off with a rise in the beam to the left (UK) or right (European).
On the ST the dip beam is a projector unit and the beam pattern and dip is done entirely within the projector. Note that headlamp glass on the left side over the projector is entirely plain. The main beam pattern is produced both by the reflector design and the the glass of the headlamp lens.
If the main beam is directed ot the right, either the headlamp adjustment is badly out of alignment (there is a side to side adjustment screw at the right rear of the lamp unit), the bulb isn't seated properly (this is easily done with H1 bulbs) or, the front subframe is bent.

That's interesting, Derek.

Those adjustment screw (8mm A/F) on each side of the lamp unit at the rear: I'd been told that they are 'tilt up and down' adjustment, rather than side to side. Are you saying that isn't the case? I'd have thought 'up and down' adjustment as all you'd normally need, after all.

Anyway, I've screwed them (outwards) till I'm blue in the face, but it didn't seem to move the beam hardly at all. In any direction.

As has been mentioned before the springs holding the bulbs in can lose their tension allowing the bulb to wobble or even point in the wrong direction.
I've also seen bulbs installed upside down which will also adjust the filament's position in relation to the reflector & threfore throw the beam way out of line.

As for adjustment...

Headlamps up to 2003
Adjuster on LHS ( near to fuses) clockwise adjusts down, anticlockwise up
Adjuster on RHS ( near to coolant header ) clockwise adjusts right, anticlockwise adjusts right.

Is it a UK headlamp?? look on the LHS of the lens as you see it, there are 2 E3 markings... at the lower part of the lens... the one towards the centre of the lens has an arrow pointing in the direction the dip beam goes... UK should point to the right as you look at the lens.

( To alter the dip if it's non UK remove the dip bulb & inside the lamp is a small plate held in with a screw... this defines the cut off for the dip. Get chopping & bending... ) It's much easier to get out if the headlamp is removed which doesn't take too long once the fairing is off.

Have you actually set your headlamp to the standard setting fiirst???

As per the manual, with the headlamp 10m from a wall, the top of the horizontal cutoff of the dipped beam pattern thrown onto the wall should be about 90% of the height of the centre of the headlamp from the floor... this could be an easy starting point.

Clockwise left, anticlockwise right? That makes sense.

Many thanks.

The arrow on the lens (under 'E3') points in both directions: it's double-ended!

Or one of each :D