The regulations on controlled assessment are pretty clear that there is no appeal available to the examboard for consideration to be given to students (whereas in an exam consideration can be given if the school makes a mess of stuff).
We had an incident a couple of years ago when CA first came in - the member of staff stopped the students working at the end of the lesson rather than giving the students a full hour as arranged. It cost the students around 5 minutes.
We contacted the examboard and they said "tough, sort it out" - fortunately this was a small CA piece and the students were able to do another a few weeks later the nature of that particular subject was that students did multiple pieces and the best ones were included in the portfolio anyway. Parents were kept in the loop throughout .
OFSTED may be interested if you felt strongly enough to contact them with a complaint - if they felt it serious enough they could spring an early inspection of the school but, more likely in this case, they would see it as a piece of information to file away and only trigger an early inspection if there a range of other, unrelated, issues coming forward.
I think the school should, at least, offer an out of school hours alternative for students to do a new task - I can understand why they may wish to focus on revision in lessons - they may judge that the gains that will give to exam results are greater than the gains from doing a new CA task.