Very little, which surprises people. Quarterly updates are explicitly provisional, and the mechanism for fixing an error is simply to include the corrected figure in a later update or at the final declaration — there is no amendment form, no penalty for an honest mistake, and no need to phone anyone. The number that actually matters is the one on your final declaration, and that is where accuracy counts. This is why we tell clients not to hold up a submission chasing one missing receipt: file on time with your best figures and correct it later. The penalties under MTD are for not submitting, not for submitting something imperfect. Getting that distinction the wrong way round is what makes people miss deadlines.