I have never had anything other than random rosters in my "reserve period" with very few (if any) sby duties. Most of the other pilots I have spoken to have been in the same boat.
Try looking on the BALPA notice board and you will see that this has been happered out on several occasions. If you are not a member, then you can join by visiting
http://www.balpa.org/scripts/members_join.pl?act=form