I’ve just listed a battery on ebay which was a replacement for my S1. Starting price is �40.
http://cgi.ebay.co.uk/New-Halfords-Calcium-Maintenance-Free-Battery-HCB063_W0QQitemZ160424187171QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item255a06d523