I think the word you are looking for is "quota".
There are approximately 18 slots available after 23.01 and 06.00 every night
There is indeed a Night Quota of 5,800 movements per year during the night period (which actually starts at 23:30). That's 16 per day on average (in practice about 14 in the summer and 18 in the winter).
But regardless of what you call it, the OP's assertion that Heathrow are seeking to make changes to the quota/curfew any time soon is incorrect.
Instead, the issue is what happens if and when R3 is approved.
The Davies Commission has recommended that when a third runway is built it should be accompanied by a complete ban on night flights. Heathrow is refusing to accept such a ban and is pressing the Government to relax that precondition so that some night flights would still be allowed if R3 goes ahead.