I have been experimenting with a 15mm crochet hook and some super chunky yarn. It was great fun and has resulted in a massive granny square that will make an excellent cushion cover. I have taken the picture with a normal double knit blanket next to it to give a sense of scale.
I used Sirdar Big Softie yarn which is lovely to work with, but it is too expensive for me at the moment so I also tried using more than one yarn at once to see how that worked.
I was very pleased with the effect of using three different yarns to give a multi-coloured finish, this technique would also work using three lots of one yarn to give a single colour effect. Now I just need to find enough yarn to make a blanket.