Picture posting is enabled for all :)
I think you check the tension by knitting a square in stocking stitch that's bigger than 10 cm square, then mark a row, count 10 cm then mark that row, and then do the same for columns. That way you'll know whether you nit tightly or loosely. If you have too many to the 10 cm you knit tightly, therefore need larger needles to get the right tension, if you have too few you knit loosely and would need smaller needles. Or knit a larger or smaller size.
Georgie, Pearsons do sell wool, I think it on the top floor, poked away in a corner. They used to have cheap double knitting wool as I used to buy it when I had a phase of knitting soft toys. Charity shops sometimes sell it. Was the one in Lancaster Road so full of wool you couldn't get in and the woman could never find anything. Not surprised it closed down.
How about doing this ?.http://brilliantbritain.blogspot.com/2008/11/help-knit-worlds-biggest-tea-cosy.html