My Novagard 6 bellbox strobe has never worked. I think this is down to the fact the installer used the existing four core bell wire from a previous alarm and didn't wire in the strobe. However, I've heard you can still use the strobe with four core if you wire it together with the bell trigger. In the real world am I likely to get any problems (fuses blowing etc) by doing this?

Assuming your panel can output or sink enough from its bell output then no

Once local round here always does bells in 4 core. Must save at least 4p a job.