Exactly the same thing happened to me with my Black and Decker GX635 mower.

I repaired mine with a narrow steel strip some self tapping screws and hot
metal Glue. Once fixed I taped a wooden stop on the main handle that stops
the switch from being over stressed. I do think that it is a design fault as
the travel on the switch after the required contact is far too much and it
is difficult if not impossible to not over stress it especially if you are
cutting the grass on a bank.

The repair is now in it`s second grass cutting season.

I had a similar problem with a B&D rotary mower - model uncertain. I
replaced the switch with a standard 13 amp plug & socket combination clamped
to the mower. Worked OK until the rotary drive fell apart apart about 2
years later. Always used with residual circuit breaker as safety precaution.
Now replaced by a non B&D mower. I could not recommend anyone to buy a B&D
mower after my experiences - on mine, the handle kept coming loose from the
inadequate too short plastic lugs that were supposed to keep it attached to
the mower.

Bevan