Imagine you work Mon, Wed, Fri and you have the opportunity to attend a training course Wed-Fri. You have to work the Thursday as you're on the course, so you book off a day in lieu later in the year, maybe take a Friday off.
Then the week of the course arrives and you are very sick and the GP signs you off work for a week. You have to cancel your attendance at the course.
What is the situation re. Time in lieu for the Thursday you were planning to work but couldn't due to sick-leave? Do you now take it out of your diary and work the Friday as normal?