The car is a Renault Megane R registration 1.6. The driver's seat seems to have developed a fault in that when I sit in it, it moves downwards. Seems something broken from the parts that hold the seat together.
One possible solution is set the seat in the correct position, mark it, remove seat drill 8mm hole through seat if possible and put a nut and bolt through to secure in that position, failing that a 2nd hand seat even with different trim could be swapped over, a local garage should be able to swap the trim over if needs be.
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