Very useful video - cheers for uploading. For anyone who might be facing the same issue I did (innacurate readings / weird fluctuations when weighing) - it was down to the 2 large screws both being loose. Tightening them very lightly just to the point where they stop turning with light torque should be enough. Don't go too tight or the scales will just error. After tightening the screws and comparing to a brand new scales reading they are both showing the same.

It's a load cell. Detects the distortion in the bar. Created by the holes

Same scales,same fluctuating weight behaviour so looks like it is a potentially common mode of failure. Not opened mine to check production date but reckon probably not dissimilar in age range to yours. Oh well, time for new scales. Thanks,was interesting to see what was inside.