if you did not set a code in it when you installed it try to use all zeros. if you did put in a code and forgot what it was there may be a reset button somewhere.
I was told by a car audio installer once that if you put the radio in a fridge overnight the code will be "wiped" by the morning. Never tried it myself, but he swore it was true!
try places near you - this happened to me a few years ago and a wee place in my hometown charged �5 to find out the code and reset it for you - was all done in 10 mins - a mechanic may know of someone who does this near you
Take it to a Vauxhall dealer - they can reset it if they have a record of the car. Otherwise they will have to send it away to a specialist decoder who charges about �20.
PS Why did you have batter in your car anyway- spill your fish'n'chips ? :-)