Peanut Mouse has excellent reviews.
It's basically an ergonomic small wireless mouse which can be recharged via usb. It supposedly has a month's usage on a single 2-hour charge. Also note there is cashback available via quidco and topcashback.
Haven't seen it cheaper elsewhere but is supposedly a great mouse. Have bought to use with my tablet.
Select the white one for free delivery regardless but to get the grey one you will need to spend £10- don't know why that is. Thanks to worthinger for noticing this.