To answer the leave questions:
Leave at LHR is two weeks a season, with a duty free week
You can take these in blocks of a single week or as a two week block, you could, in theory, put the DFW up against the leave block of two weeks.
The leave blocks have 3 wrap days attached to them meaning that you can make the week into 10 days.
Leave and the DFW attract the daily rate of CAP. So divide monthly cap by the days in the month and you attract that much credit per day on leave and DFW
Wrap days and DFWs can be worked in, leave cannot.
Leave is bid for but is not seniority based. Each leave block has an attached value in points. This adds to your pot. More attractive leave blocks earn less points. When the leave bids are allocated, the highest points gets the leave allocated first.
Points last for (i think) 8 years, so after that time the points you earned 9 years ago are wiped - but the points for the last 8 years remain.