You can't, you should have read the small print in my disclaimer...

That depends on what the nutrient load is like in the tapwater. Water changes are all well and good, but if the water is packed full of nitrate and phosphate the OP will go around in circles.

100mg/l. is a very high amount of nitrate, are you absolutely sure that's the correct reading? Nitrate test kits expire very quickly (within 3-5 months) due to the the short shelf-lives of the test reagents. It wouldn't be a bad idea to get a second nitrate test kit and see what that brings up; an aquatic store would also be able to test it for you.

Personally, I'd keep an eye on the fish for the next 24 hours before adding any treatments. If the problem is Whitespot, then the spots will become even more obvious before the 24 hours is up. Going by the photos you've supplied, it doesn't seem like the fish is suffering from Whitespot - but then again it's much easier to diagnose a problem when the fish can be seen in the flesh.

My money is going on either Lymphocytis or a bacterial infection.

Also, I should have mentioned that you can purchase nitrate-reducing resins for aquarium filters. The most well known of such products is API Nitra-Zorb.

However, as I said earlier, it depends on the nutrient load of the tapwater. Very few areas of the UK have tapwater that is of what you could term 'good quality'. If the nitrate level is critically high in water from the tap, the only way to get around this is to either filter the water (through an RO or Nitragon III unit) or to buy RO water from an aquatic store. With the latter option, the water may need to be tested periodically for TDS and nutrients, as well as requiring re-mineralization to restore the pH, DO (dissolved oxygen) content and essential trace elements.

todays update : she has less spots on her, mostly on her tail now and less raised. Her eye is now just cloudy rather than looking flakey like it did last night. Shes not rubbing on anything that Ive noticed and is swimming about quite happily now.

I will try and get a better pic of her today-not easy when several platys swarm round when I get near the glass!! Will try with the long lens on and sit over the other side of the room

Ive done a partial water change and temperature is currently up to 25c and will turn it up a little again shortly. I will buy a new water test kit this week just to double check xx thanks for all the advice you 2 xx

Just wanted to update: the following morning I came down and she was struggling to swim and was bloated-I suspected dropsy and when I came back from work she was on the bottom unable to swim far and being harrased by the other fish. On moving to a breeding tank I noticed her eyes had become very swollen and her scales had that pine cone look about them. I went out to buy some more clove oil to put her out of her misery but when I got back in (20 mins later) she was gone I couldnt believe in the space of 24-36 hours she went from ok to gone.

Thank you both for your advice though xx