Canna makes good stuff, but it's pretty spendy, there are a lot cheaper alternatives that are just as good. i'm not sure whats available in england, though. a lot of people here are trying out a new vendor and liking it i guess, megacrop from greenleaf nutrients.
a lot of other people here use Jack's (or other) dry salts, and mix in varying amount of calcium nitrate and epsom salts through the grow.
i'm a jack's guy myself, a whole years worth of nutes for me is maybe 60 bucks.
as far as mixing the mutes, they'll come with instructions. the only one you shouldn't follow is how much to use. almost all nute companies will tell you to use quite a bit more than you actually need to, they don't care what you're growing, they just want you to use up their stuff so you have to buy more. so cut the recommended amounts in half to begin with, and work up from there. always can add more, taking extra out is a lot harder.
if they say add part X first, do it, they coat the nutrient salts in chelating agents, (often amino acids) so they don't react with each other, and can be in the same container. if they say mix in one or another part first, its so it doesn't mess that up. you can add epsom salts to water that has calcium well dissolved in it. if you add it to water before the calcium is completely dissolved, they'll bond with each other and precipitate out of the mix, so your nutes now have very little available calcium and sulfur.
so just follow directions, cut the suggested amounts in half, and watch...