Allbatteries monday only sale 30% off with CYBERMAN code. Had one of these 3 years ago and it's still going strong. They also have Yamaha & Bosch ones but the 26V batteries seem best value (£ per WH)
http://www.allbatteries.co.uk/li-ion-battery-for-electric-bike-panasonic-type-26v-17-6-ah-458wh-aml9121.html
http://www.allbatteries.co.uk/li-ion-battery-for-electric-bike-panasonic-type-26v-17-6-ah-458wh-aml9121.html