The low beams are poor on both headlight units - night driving is pretty scary -and the Croma failed the MOT today partly because of this. They were not only misaligned (after me trying to adjust them before), but the actual beam was "diffuse". The reflectors look slightly spotted, but not terribly worn, and the outer plastic casing is slightly, but not terribly, scratched.

I can´t really tell if the problem has appeared gradually, I remember noticing the poor headlight function a few months ago and hadn´t thought about it before, which makes me think it came pretty quickly. I changed the left low beam halogen bulb about 1,5 years ago, then at the previous MOT it was misaligned but they corrected it for me with the adjustment screw (maybe wrong approach, should have reinstalled bulb, it might have been askew in the reflector...). The right bulb has not been changed for the 3 years I have owned the car.

I should probably double-check the bulbs are ok and correctly installed within the reflectors, and perhaps try to polish the outer plastic, but if that doesn´t solve the problem, can you think of any there is no other remedies than to change the entire headlight units?

The main questions here are:
1. Should you rally expect the reflectors to wear during the lifetime of the car, if the correct bulbs have been used? It is a 2005 with 130 000 km on the odometer.
2. Will a worn reflective have clearly visible wear, and not only small spots on them? Can upload images if it helps.
3. Is it very difficult to change the entire headlight units by yourself, i.e. does it require dismantling the entire front plastic bumper (maybe this is in the electronic repair manual, haven´t been able to check yet)?

Thank you in advance!

@Cromatic
It's unlikely that the headlights would fail for damaged reflectors or lenses unless they were visibly pretty poor. the bulbs are awkward to fit and easy to put in out of alignment. The metal fitting that the bulb sits in is easily displaced in the plasttic moulding and this an cause misalignment even if the buld is located in it correctly. First thing to do would be to get two new bulbs of known good make (Hella, Philips, Osram, Ring etc) fitted and the alignment adjusted.

Changing the light units requires removal of bumper, grille and wheelarch liner according to eLearn.

Robert G8RPI.

Thank you G8RPI! Helpful information as always

Hi Chromatic,
I had to change my NSF headlight a good few years ago now following an altercation with a deer. It was very fiddly and as Robert says some of the fixings are inside the wheel arch. In decent light (I changed mine in the dark) assume that it will take a few hours to remove all the nuts, bolts etc, most of which will be rusted on. I'd definitely recommend having a dremmel or similar handy.
Definitely check that the bulb is in the correct way up - I had an MOT failure for lack of beam pattern on the drivers side once because of that - first and if there is scratching on the headlight itself spend the £15 or so on a headlight restoration kit from a supermarket.
I had a Mazda Bongo for a while and the lights on it were terrible. Apparently the Japanese put coatings on them to reduce dazzle and at night it was like driving by moonlight! I tried various YouTube "hacks" and wasted loads of time in the process but it's up to you if you want to try that first. I'm sorry I wasted my time anyway.
Any issues post back and I'll try and help.

Also osram NightBreaker Unlimited or similar will make a massive difference. I had my croma with NBU bulbs fitted parked next to a colleagues Isuzu pickup with standard bulbs and mine looked twice as bright. The difference was amazing.