Test Kits:
I, (and I'm under the impression that most people on here are the same), use the API Freshwater Master Test Kit. Obviously it's up to you what you buy, just make sure you get a liquid based test kit, not the 'test strips' as they are notriously inaccurate.
Ammonia:
Never heard of using window cleaner, in fact I'd be worried that might have things other than ammonia in it.
As you are in the UK best places I have found to get ammonia is BOOTS, or Homebase. For BOOTS it needs to be a fairly big store as it is sold with the cleaning stuff, and the small stores tend to just stock the chemist & baby type things. In both shops it is called "Household Ammonia", in Boots it's a normal round white bottle with red lid, and in Homebase it is a squarer white bottle, (but again labelled as household ammonia).
Whereever you get it, check the back of the bottle, and the 'ingrediants' should list only water and ammonia, or perhaps it will say "xx% ammonia solution", or even "xx% ammonia w/w".
Really getting the ammonia is the BEST way to do it, as then you can accurately control how much you are putting into the tank, whereas with using flakes etc it's a bit hit and miss.
On the topic of water filters my water filter specifically says it should NOT be used for aquatic use.
I think it is because of the way it can destabalise your water conditions, (lowering the Ph, and also lowering the hardness, and hence the 'buffering' ability of the water). This is the thing I would be MOST wary of, as unless you have fish that actually REQUIRE softer water, lowering your KH/GH is not something you normally want to do)
Brita just advise against it, but don't really say why, perhaps just covering themselves:
Brita Advice
