I thought I should come and update this thread, as I chose to cancel my order for the paperwhite. I suddenly realised that the 2 important aspects were the smaller size and the fact it was lit, and I also realised that for a little more I could have a tablet, with the kindle app. I therefore decided on the 7 inch Samsung Galaxy Tab, which I now love. The kindle app works well on it, and the brightness can be turned very low, so it suits me fine.
For those of you considering a Kindle Fire, I discovered during my research that you're limited to the Amazon app store, and can't use the full Play app store, which I think is a shame. (Plus, if anyone is comparing tablets, and is put off by the lower storage in the Galaxy, it has a slot for a Micro SD storage card. )