He is a (now retired) returning officer and he told me this:
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All spoilt papers are separated and declared “doubtful ballots”
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Prior to the final count result being announced, ALL candidates must review ALL doubtful ballots. The reasons are two-fold ensuring every paper is accounted for. It also provides candidates the chance to look at the nature of the spoilt papers and the messages written on them as they desperately try to claw back some votes.
It causes a LOT of issues marginal seats where recounts may be demanded. Those candidates want EVERY ballot paper scrutinised closely again.
So although one individual spoilt ballot is not reported on, if large numbers of the electorate spoil their papers, it will be national news (as was the case in France a few years ago).