Are you sorting your whites from the colors? Synthetic fibers pick up color very easily, especially Nylon. Nylon that is sturdily constructed can also take hot washes, up to 140 (60C). If you are indeed sorting, then it is possible that your white nylons are gray because of body soil that is not being shifted out of clothing due to cold washes.

Have you tried an oxygen bleach with the detergents? Even when I use a detergent that claims to have oxi-bleach, I have to add more in order to keep the white clothes 'white'. Otherwise, our whites get rather grey. I have some synthetic sports garments that, in spite of my best efforts, insist on remaining grey. :-p

Nylon and poly should not shrink, so there is no reason they cannot be washed in 104F (or 40C), which is a temperature that is warm enough to activate the oxygen bleach so that it will work.

Thanks for your quick responses.

@Miele1966. I always sort the whites and colours, and the only things that go in are pure white - the whites with logos etc. on go in with the lights - and they are all fine. I will try a hot wash

@Cavimum. I will get some vanish Oxi-powder at the shop tomorrow (Uk equivalent of oxi-clean) I will use a 60C wash as well

It took a while for the grayness to develop so likely will take more than one "power wash" to have an appreciable impact on clearing the problem.

I second what Dadoes said. A long soak, with hot water, and a hefty dose of good enzyme detergent might make that happen faster. Then re-wash and rinse thoroughly. Try borax, too, if you can get it.

OP, I would be reluctant to wash synthetic fabric, or cotton/synthetic blend, in hot water.

I might try a long soak in warm or cool water, and oxi might help you with that. A lot of people prefer hydroxen peroxide for use on white clothes in warm or cool water. It is very inexpensive too.

I will try washing in Hydrogen Peroxide. How much should I use. My machine is small (6kg, so about 2 cu.ft) It is a front loader. I bought some UK Persil and tried that, it made a small difference but not exactly ground breaking. The labels suggest that non-chlorine bleach can be used too. I will try the HP first though. Thanks all again for your helpful advice!

if they are already gray, it may too late to make them white again. when you buy new clothes, try oxi-clean (or similar) powder along with a good detergent, in EVERY wash load for your whites. I've got all sorts of white clothes, socks, etc that are pristinely white because I use oxi-clean along with a good detergent. good luck!!

Try about a 1/4 of a cup of the Peroxide, along with your normal dose of good detergent. The warmer the water, the better, but not necessarily 140 degrees. Again, it will most likely take numerous washes to release all of the grayness. I do laundry for my aunt, who has tons of nylon and other synthetic materials. Over the past year or so, I have managed to get them all white again, after years and years of cold/cool washes and cheap detergent. It will take time, patience, and trial and error. If you truly love the garments, then it is worth the time and effort, IMHO.

No remedy is guaranteed. I've got some white athletic socks that are terminally dingy. Sometimes the graying is permanent. Good luck!

Thanks all for your help. I've solved the problem. The clothes don't look new but they are much better and it should get better over time. Here's what I had to do
1. Prewash in warm 104F/40C water with a full dose of detergent and 1 scoop of oxi-clean
2. Wash in hot 140F/60C water with 0.5 cup Hydrogen Peroxide and a regular dose of detergent
3. As many extra rinses as your washing machine will allow.
4.Hang-dry the items, then run them in an air only cycle in your dryer for about 10 minutes to soften them up.

Once again thank you very much

Lemon Juice in a spray bottle. Then hang in the direct sunlight.

PS Do not dilute the lemon juice. Use a real lemon and a reamer. Strain the juice before putting it in the spray bottle. Hydrogen peroxide works, too.