I bought a good Sony Ericsson Xperia Ray on
eBay for £70. It's a small but nifty Android smartphone but they do not hold their price as well as more popular bigger phones so you can find them cheap. For example,
this one might be worth
a punt?
It is a great size for carrying in a pocket when cycling or walking. The screen quality is really good but it is small so you need reasonable eyesight and it is best to use a stylus on the touchscreen unless you have tiny fingers.
I am another happy giffgaff customer.