Think i might have got to the bottom of it. Went into my LFS this morning with a water sample, explained what had happened. Ran new plants under the tap then put them in the tank then the cherries played up. Did a water change, left it 2 days and tried again and it played up again. His words: "was one of the plants cabomba" Yes "did you get it early in the week" Yes "Did the plants look really super fresh and come in neat 'flat' plastic bags" Yes He basically said that the cabomba before it leaves Singapore it MUST by law be sprayed with insecticides before it is allowed to leave the country. These plants then when they arrive in the UK are washed for about a week normally before being sent out. When they are washed they are in big tanks that have water flowing in one end and then flowing out onto the floor the other. So they are really thoroughly washed. I also know the place I bought it from said that on average they hold plants for less than a week before they arrive at the customers door to ensure they are 'fresh'! so how on earth did they wash them in that time? as they are meant to. Apparently its fairly common for cabomba especially to really hold onto things as its got so much surface area and even running it under a tap wont wash it off as it clumps together and becomes a little sponge like, its not the first time he has heard of it. He said don't worry about it now if the other things in the tank are not bothered by it just add carbon to the filter ( which I have done) and do water changes of about 50% twice in the next week and then try again. He said he would eat his hat if it wasn't insecticides. I trust his opinion as he used to work in a massive wholesalers and used to visit the farms where this stuff is grown saw them being sprayed, and then overlooked them being washed this end. SO from now on I will be soaking every plant for a week or so before putting it in the tank! unless i know they have come from a friends tank or w/e and then I will rinse under the tap. So this time next week we will see if he eats his hat i guess makes sense to me though. The reseller is on ebay, and the plants are really good quality look like brand new perfect condition so i would recommend them for sure, just be sure to wash and soak for a long time!