I recently added a fourth lens to my kit and had to get a larger bag. I chose the Lowe Pro Flipside 400 and have not regretted.
It takes a little getting used to but it's main feature is that you can keep the waist strap on, take of the shoulder straps and swing the bag around in front of you. You access the insde of the bag from the back. Standing in water this is a huge advantage.
That said, i do not have a lens as long as the 250. my lenses are 40, 50, 80 and 150 a 503CW and 3 backs.
There is room for filters and small stuff in front of the bag and i have bought one of the lens attachments which holds my extra film rolls.
It is possible that one of the many lens attachments would hold your 250mm.