As far as i know the BAA check the scales every night.
When scales are broken a nice gentleman comes round to fix them, and it was he who told me they SHOULD be checked everynight.
But what you should do and what actually happens are two different things.
Whilst i appreciate excess is expensive, especially to Shanghai, if you came to me and asked to see the 'certificate' for the scales before paying your excess you'd get an offer to check it on the next scales. Failing that we'd probably end up arguing that unless you pay the excess you're not going anywhere.
Most check in agents know when their scales aren't working. You don't have to be in this job long to be able to accurately guess the weight of a bag without needing to put it on the scales.