Any idea why the starter cord on a petrol mower should jam, when
nothing has been caught in the mechanism? The mower was working fine,
then cut out, because it was low on oil. We filled the oil but the
cord now jams and won't pull.

I'm sure this has happened before and after a while it worked OK.

Is there some built-in fail-safe that causes this?

If possible, can you reply by email as well?