A PC680 start at about £80 on ebay, or an equivalent non-branded version of the same is £59.99 on ebay (Only difference is the terminals, but the Odyssey only has threaded holes as standard anyway)
http://www.ebay.co.uk/itm/WESTFIELD-CAT ... 1c23cd0183
A standard new battery is going to cost you the best part of £50 anyway if not more, so...
This is assuming you're sticking with the lightweight spec and not running a big-ass stereo and subs or anything?