Interesting Will, I had the same issue with the tailgate on my series 1, intermittent refusal to open and had to be freed up using the method you described.
Eventually all the harsh treatment resulted in the breakage of the little plastic linkage / hinge between the metal rod which comes from the servo to the one which actually operates the lock. A 'fix' with cable ties was unusually less than satisfactory and resulted in intermittent failure to lock / unlock due to not being the correct shape and working the rods slightly out of position over the course of several lock/unlock cycles.
So everyone, take Will's sound advice and lubricate yours before it seizes up and breaks something, or at the very least leaves you struggling with the boot in pissing rain when you desperately need to get something out of it!
Re the BMWs, funny isn't it, I find that a lot, they actually seem offended or something that anything has overtaken them, let alone an old Shitroen.

I quite regularly encounter the Luftwaffe happily pootling along motorways or dual carriageways at 10-15mph below the speed limit, but the moment
anything starts to overtake them they either accelerate, or allow the other car to complete the manoeuvre then immediately overtake it, before settling back down to below the speed limit! Have seen this go on for miles sometimes.
Ciarán