You don't say if you selected the Alternator off, and pulled the field circuit breaker.
Full marks if you did. If not, something to revise for next time! As Fujiflyer says, alternators can fail in several different ways, but in all cases, the failure benefits from being isolated completely. This minimises the risk of fire, or damage to other electrical equipment.
Alternator failures are relatively common in GA types, so this is worthwhile understanding.
Diode failures are not normally instantaneous. Usually, they follow a period during which you will have had a noticeable background whistle on the intercom. If you are suffering in that way, try switching the alternator off momentarily and seeing if the whistle goes away. If it does, expect an alternator failure over the next few flying hours and get it looked at.
Well done Ringway Flyer on a safe landing.
TvR