Are the lenses glass or polycarbonate ?

These will be polycarbonate Lens. Plenty of aftermarket refurbishment kits available. 3M do a good kit , about £25 . You’ll need a drill or suitable power tool for the rotary mop

I used that very kit only last week, had been polishing mine every now and again with metal polish but still looking yellow on the top section, after using the 3M kit they now look like new...however after using the first grade of glass paper the frosted glass effect had me a bit concerned for a few minutes, all came clear subsequently.

The thorny question is how is best to keep them clear afterwards when the sun again does its damage, i've used a glass sealer polish followed by a couple coats of wax polish for now, a chap i respect has advised using CapPlan No1 Supergloss couple of times a year (for whole car too), he lives in Cyprus and has the same car as i and this keeps his lights and paintwork protected so i'll take that as read, others will prefer other methods and products.

Thank you very much for the advice. I’ll order a 3M kit.
It's time to spend a bit on the old girl, I’ve had her nearly nine years now and the itch for something different scratched recently but the overpowering reaction from the family “Dad you must be mad “ :-) restored me to my senses. She’s never been out on salted roads and is remarkably solid underneath. Apart from routine maintenance I’ve only had to pay for four springs, a cam sensor and an air con compressor. A Toyota would probably have cost more to maintain.
Next it will be the rear wheel arches then a wheel refurb and a professional valet/restoration of paintwork. Then keep her till either the car or me hits the buffers.

Thanks again.