12 volts on a battery means naff all :bang:, it's the capacity of the battery that needs checking, by drop/load testing it. Any competent garage should be able to do it. What normally happens is when the starter is engaged, the volts drop right down, normally associated with a duff battery.
It's a bit like saying there must be fuel in my tank because I can smell it, doesn't mean there's any usable amount in.