One thing Barry suffered from when purchased was yellowed headlights. I tried the quick fixes like T-Cut and Xerapol plastic restorer, but whilst they improved them, they still looked pretty poor.
So I bit the bullet and bought the 3M Headlight Restoration Kit. It cost £22 (not much more the T-Cut and Xerapol cost me!). You can't get replacement lenses, so this is the only way to go really.
The kit contains everything you need including the masking tape (you get more than the picture - I took it after finishing). All you need it a drill. If using a cordless drill make sure you have enough battery life to complete the job! I used 2 batteries on my DeWalt. Picture doesn't show the sachet of polishing compound as I'd opened that and it was going everywhere
It is well worth watching the 3M video on YouTube before starting. Search for 3M Headlight restorer.
This is how the lens started - all yellow, misty and pitted (and this is after T-Cut and Xerapol!):
First stage is very scary using the P500 disc. This takes off the surface of the headlight. It took about 8 passes of the drill. I used two of the P500 discs provided (the kit comes with 6). Care is needed around the indicator bulge. If you press or linger you get little bits of melted plastic build up. Not much pressure needed! In the end I did round the indicator by hand with the disc. It's worth wiping off the dust frequently as that will also melt if you let it build up. To get to the edges, especially the back ones, I made sure there was a good layer of tape as you have to move the disc onto the tape to get right to the edge of the lens. Carefully does it! Make sure you take off all the pitted yellow layer - keep wiping with a cloth until it looks evenly milky white.
Next you move to the finer P800 disc. Again, about 8 passes taking care to cover off all areas. A bit fiddly round the indicator - again I finished by hand here. Wipe regularly to so you can see coverage. Aim is to get rid of all coarser scratches. End result looks like an even white.
Third step with the "Trizact" P3000 disc. This needs to be kept wet. Use a plant sprayer to wet the disc and lens. Then go over the lens evenly about 10 times. A white slurry forms. You know it's working then.
Final step is the polishing compound with the orange sponge like pad. Put a good pea sized amount on the sponge head and smear over lens before starting the drill. Polish all over the lens until the polish is removed. Wipe down with a cloth and go "Wow". Lens is like new (the apparent swirls in places in the picture are in fact reflections of the tree behind).
The final step was to apply a surface sealer and UV protection coating. I bought this from Amazon and hopefully it will prevent future yellowing:
https://www.amazon.co.uk/gp/product/B00A3S1RGU
The headlights now look amazing. Well worth the money and an hour of graft, if a bit scary to undertake!
Let down a bit by the fog lights now - they have been well and truly sand blasted. As they are glass, I'm not sure what to do other than replace them.
I'll put this into a fact sheet if people would find it useful.
It's taken years off Barry 
