Accenta 6 alarm - alarm bell not sounding outside?

Our alarm went off for no apparent reason the other day & putting our code in didn't stop the external bell sounding. In an effort to turn the alarm off, my husband flicked the fuse box switch to the alarm but this had no effect other than to put out the lights on the internal control panel. When we flicked the fuse back on, the lights came back on the control panel & nothing appeared wrong, no 'tamper' or 'attack' light came on. The 'power' & 'day' lights remained on. The alarm sounded for 20mins & then went off. Afterwards, the key pad seemed to work as usual. When we tried testing the alarm though, we found that when it triggered, the bell indoors could be heard but the external bell was silent.
We've only recently bought this house & whilst we were given the alarm code, we weren't given an instruction manual so don't know where to start trying to work out what is wrong! HELP!