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.
Thanks for the reply. I'll suggest mum takes it back to the dealer again to fix the fault. It only happens in wet weather - so sounds a very similar problem.
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