You will join on a 75% contract straight away if you dont have previous jet experience and optional winter leave options are available immediately as far as I'm aware, you have to bid for OWL but I don't know anyone who dosent get it unless you are a trainer, then it gets a bit more complicated. They have just introduced fixed part time contracts of varying length, with all of the time off over the winter, but you need to have at least one years service to apply I believe.
They are also mandated to discuss flexible working options for people but you generally need a VERY good reason to bag one of those, generally some high level family commitments such as caring for a parent/child.