If the joint box is going to be fixed securely to a wall, and you enter the 2.5mm2 via a regular 20mm plastic compression through the BOTTOM of the box, I can't see a major problem.
Sorry for the late response, it has a flat cable going straight into teh back of a led outdoor light at the moment, its been like that for nearly 3 years, i may look into the easist option as recommended by sparkwright.
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