Some of her income has tax deducted at source and some doesn't. Instalments are paid as requested by the Revenue each January and July against the expected balance of tax due. These twice-yearly amounts have been identical for several years and my mother's income has remained fairly constant other than some increase in line with inflation.
A few days ago I got a reminder from the Revenue that it was almost the end of January and that the next instalment would be payable by then - but I had never received a statement telling me how much to pay. I phoned the taxman and was told that the January instalment would be over six times the regular amount. I asked for written details of their workings and they arrived last night. After an hour or so this morning I worked out what the Revenue had done (or rather, NOT done) and phoned them up again. Result! The instalment due is now back down to the normal amount and written confirmation is in the post.
What worries me is this. How many other folk get tax demands that are out by a factor of six? How many of them don't have access to someone who can calculate the correct sums due? How many elderly folk become sick with worry on receiving outrageously wrong tax demands?