battery power

[rs20004wd]

 I run an Adam Harper 'Turbo Conversion' 24V C5. I'm looking for a battery upgrade for reliability, longevity, weight reduction and to make use of the latest battery technology, but whilst keeping costs down too. As the current batteries may have seen better days, but still good, I need reliability as I use the C5 to commute to work ( I do get some funny looks and people break there necks to stare whilst driving and then pip!). When I get home the current pair both get charged ready for next time. I use the Optimate 4 charger.
I've seen these on EBay  (Item number: 110867835984) but £400 for the pair is extortionate. What's a good alternative?

[marra]

Used regular for customers e-bike builds :

http://www.pingbattery.com/servlet/the-24V-LiFePO4-Battery-Packs/Categories

Li Ping can build whatever size/dimensions, capacity you need. M