atco-matic,
Not sure it's quite as clear cut as that. There will ALWAYS be the requirement for the D1s as they have to open sectors. It is also preferable that some of these are west enders to relieve those that have been on all night-after all thay would be the first to complain if they were never getting a relief or extra staff to split. The number of D1s is no different to what we have now as far as I can see.
What it does do is account for most of the staff who will not be working a 6 on 4 off cycle and so the vast majority of the rest of the staff will be doing just that.