Not so Crab. We change over crews at 1300L. If we get a night job we still return to RS15 at 0800L. Same cover as supplied by MOD.
If the crew is unusually fatigued the Captain may, in the interest of Flight Safety, down declare. But that was the same in the Mob, no? This is obviously a rare occurrence and I have not seen it happen.
If we go night training however, different rules apply. Then we come back into work 10 hours after we land. So in that case RS15 may not be resumed at 0800L. Why the rules are different I do not know, but they are.