dog baitThe flexibility part of the agreement differs from office to office so unfortunately you will have to get a copy of your local agreement and check it, normally it involves working elsewhere, on other duties and startiing and finishing earlier. The sickness and leave bit should form part of the summer lapsing agreement which again you will have to check the local agreements.
Don't forget to check the rules out at http://www.royalmailchat.co.uk/forum/rules.php and of course enjoy yourself.
flexability is where you may be asked to vary your start or finish times by up to 30 mins a day but you would not work more then 40 hours a week,say for example a van bringing work to your office is running late then the manager will ask you to flex up so that the mail can be delivered.There is also a set of Enablers which deal with issues like longs and shorts,working in nearby offices etc which your local rep would have sortd out.The big issue about covering for one another comes under phase 3 Transforming the way we work and should be fun 
We also do not have an agreement yet on phase 1+2 and blame it on local management as they do not know what they are doing.