Mr Mustard really doesn't like such vague answers. "Short term" is not specified as a period and letters were not sent to three residents of my street (out of about 40 with cars) who had suffered reminder failure which makes the number of 20 seem risible. (Did Capita pay any compensation for their short-term failure? - probably not, as the council saved postage and gained PCN income so had a profit, not a loss).
Reminders have been sent ever since the first CPZ was introduced in about 1987 and so they are custom & practice, thus residents have a reasonable expectation that they will be reminded on which they are entitled to rely.
See also this FOI response to Mr Mustard
so the council guess at 20 when 46 reminders are needed per working day to equal the annual number of permits in issue of 12,000.
Anyway if you are one of the affected residents you can email Richard Cornelius at email@example.com and cc firstname.lastname@example.org with your PCN number and ask for it to be cancelled and if you have paid one, since mid-May when the process was moved to Capita, ask for a refund as you have paid under a mistake.
An honest council would refund you. Let's see how honest Barnet Council are (not very is Mr Mustard's guess).