This is a known fault with Rover 200/25/400/45's as well.  The rear door
lock barrel is mostly plastic but the actuator arm on the rearof the barrel,
held with a roll pin, is metal.  What was happening was that water would run
down the back of the car and some would go through the lock and rust the
actuator arm.  The rust would make the arm stick around half way through
it's operating arc and the tail gate lock would be half secured because of
this.  Bloody disconcerting when you are driving aong, the bodyshell flexes,
and the tailgate pops open.

I cured the problem by removing the lock, knocking out the roll pin and
carefully removed the actuating arm.  The rust was sanded off and the whole
thing re-assembled with LM grease over everything which cured the problem.

I suggest you inspect the whole lock assembly, the actuating mechanism and
the way it interfaces with the boot lock unit.  Ensure all parts move as
they should, any signs of rust which could cause the mechanism not to
operate correctly needs to be addressed.

Rdgs

PDH