Dyson - unfortunately, I am not from the UK so hopefully someone will jump in on the products you got - as I am not sure.
Ideally for each water change you will need to add a dechlorinator - possibly the aqua plus ?? It just takes all the harmful chemicals out of your tap water making it okay to use after a water change.
Ideally you need a test kit that tests Nitrite, Nitrate, and ammonia - the most widely used one (and best according to most the folks on here) is the API Master Test Kit. It would be good to pick one of those up at your earliest convenience.
As for feedings, I generally feed my fish a small pinch twice a day - for you, a very small pinch as guppies are pretty small. You should feed as much as they will consume in just a few minutes. It is okay - and generally recommended to fast them 1 day/week. As we know, fish in the wild generally do not get a morning/noon/dinner type of meal!!
HTH and hopefully a UK'er with some more knowledge then myself will hope on and give you more info about the Aqua Plus and Cycle!