The Sainsburys flour you want is the 'Taste the Difference' Canadian high-protein flour, I think its 99p for a kilo bag.
I have used it before and its a good flour as is the Waitrose one, but I mainly use a Canadian commercial flour that is sent to me straight from the miller as I am starting a pizza shop. I am currently baking my pizzas in my 240 degree C electric home oven.
I would highly recommend a stone mate, there are people on here that know serious amounts about what stone material is best, member Scott123 is the stone king!
Dont forget to post pics of the pizza you make