Robbie, if the soil is heavy and clay-like, then sharp sand (from a builders merchants, or somewhere like wickes) is good to start, as it will help with drainage, and any organic matter, like well-rotted farm yard manure, or compost, or cheapish grow-bags is great. With JI, what number did you buy? As a rule this is how it should be used:
JI number 1 seeds and cuttings
JI number 2 potting on seedlings
JI number 3 mature established plants.
When I first started out I bought John Harrison's veg growing month-by-month, it was about a fiver from Amazon, and worth every penny. It explains stuff that I complete beginner (like I was a couple of years ago) can understand, and tells you what you should be doing in your veg plot every month (although this will vary from area to area, and in years like this year, everything is about a month behind because of the awful winter we had). You can sit and read cover-to-cover, dip into it at will, or look stuff up. Is the best gardening book I've bought yet. It explains about plants that like acid or alkaline soil, how to test your soil, adding lime to increase it's alkalinity for things like cabbages, caulis and sprouts that like it that way.
A good tip for blueberries (and anything else that likes acid soil, like azleas and camellias) is to water with cold black tea if no rainwater available - tea is acidic and does contain other nutrients that they will appreciate. Next time you make a cuppa, either empty the teapot (when it's cooled down!), or if you're a heathen like me, put the used tea-bag in another cup, and pour hot water over it and let it stew. Make sure you explain to your OH that it's for blueberries - I've had him drink it before and ask why there were 2 teabags in the bottom and it tasted stewed!!