If I read your response correctly, you asked what I was referring to when I talked about dechlorinators and dechloraminators. These are products to add to your tapwater that will neutralize the chlorine or chloramine that your local water company uses to kill bacteria in the water as they are pumping it into your home. I do not know how it works over there, but over here you can call your local water supplier and find out which one they use to treat your water. You should do this because the chlorine and chloramine are both extremely bad for your fish, and because it takes different chemical products to neutralize each. And you definitely should be using these products. They are not expensive, and are available from almost all pet stores and on-line sources.
Having said that, it would seem that your problem may be coming from a combination of not treating the water properly, and the way you are filtering. If I understand your reply on the filtration question, it sounds as if you are using an internal box filter with floss and carbon. If this is the case, first you should be rinsing the floss out in tank water instead of changing it, as it contains most of the nitrifying bacteria that you need (please, tell me it's just some sort of nylon foam and not spun glass like they used to use many years ago). Second, however, that type of filter is not efficient enough for the relatively heavy bioload you have in that tank; you should probably look at adding a hang on the back filter (an Aquaclear 300 by Hagen with a couple of sponges will do quite nicely), to provide backup filtration and keep your nitrogen cycle going strong when you do find it necessary to replace your filter floss in the internal filter (it will get too clogged to rinse after a month or two, even if you are cleaning it as I mentioned above).
It also doesn't sound like you have a reference book handy that provides you with the basics on aquarium maintenance, etc. There are a number of good ones out there, none of which are very expensive, so I suggest you do a little research and get one from Tetra or Dupla or whoever sells them in Great Britain. Amazon.com should have a good selection.
I would get the water treatment thing figured out, and do 35% or 30% water changes every day or other day for the next couple of weeks, because the water parameters you mentioned above were definitely unhealthy for fish and could lead to gill burn, bacterial or viral infection, and other health problems down the road if you don't resolve them quickly.
Good luck.
