Tiger Mate,
This is only conjecture, but I think that if an individual transfered to Pay 2000 with the potential to receive an increment, that individual would have also been allocated an Increment Due Date (IDD) which is the anniversary of the date of paid promotion to that rank. That IDD would then remain fixed in that person's records to be utilized when required. An example would be a MACR who transferred to Pay 2000 Level 6, rose to Level 7 and was then capped for a while before transferring to PAS. The IDD was ineffective while he was capped, but it was automatically in place once increments became possible.
Conversely, a sgt or FS receiving MACR's pay (pre-89 reserved rights), on transfer to Pay 2000 would have received a few pence a day rise to line up with the top level but would not have been allocated an IDD at that time, because he couldn't possibly receive an increment in that rank. Upon transfer to the PAS on 1 Apr 04 his pay was decoupled from his rank and he became eligible for increments, so that date became his IDD.
This should account for IDDs of 1 Jan and 1 Jul (normal promotion dates), 1 Apr (transfer to PAS for those without existing IDD) and other dates, which should be the anniversary of paid acting rank.
Have a look at your pay statement for Apr 2001 and check the info box at the bottom for an entry, "Increment Due Date". If there is a date there it should also be your IDD for PAS. If there isn't an IDD you should have one printed following any promotion since then. Or, you will have it printed as 1 Apr on your Apr 2004 pay statement
Hope this helps....
DP