So after what has seemed like the longest project I've ever crocheted, though certainly not the biggest it is done:
I am pleased with him, and I enjoyed playing with the squares, but the sewing together of all the squares took a long time, and then creating a back for the cushion seemed to take even longer. I simply used double crochet (American Terms) all over the back in the same colour, slip stitching to the sides. It was dull and it took a while, especially when I wanted to create a sort of flap, so I had crochet some elements of it twice. Sigh. Here is the back.
Looks smart don't you think. And here is the edge, where I slip stitched to the granny squares.
So as a learning experience, although I would use this method to create blankets/cushion covers again, I would use smaller cushions and try to use more of the same colour, so that at least the sewing together wasn't quite so tedious!
Off to find a new crochet project now...