You can find manuals here: https://www.worcester-bosch.co.uk/installer/literature/discontinued-product-literature/discontinued-gas-boiler-literature/greenstar-24-28i-junior-discontinued-july-2013
I do not think the boiler uses batteries (or even batterys).
Are you looking at the timer itself? WHat is the make and model of that? Just do a Google search
X, August 2015