If your engine doesn't crank you can probably move onto checking other things after the battery.
The starter motor - check for power to it, that the earth connection is still there and that the starter motor turns or isn't jammed.
The starter inhibitor switch can cause failure to start if it becomes faulty. Sometimes, jiggling the gear selector lever a bit in N and P works, but not always. The switch is on the gearbox and is to prevent the engine from cranking in gear. One of our previous 124s had one replaced, before we acquired it.