Saddle security is an issue in Hackney. Here is an instructable to show you how to use a length of old chain and tube to insure you have something to sit on after locking your bike up.
Loop the chain around the rear triangle and your seat rail, figure out the least amount of chain necessary. A little loose is much easier to install than a little tight, so if the distance is kinda between chain links, leave an extra link in. Loop the chain over the seat rail, but leave one end hanging a few inches above the rear triangle. Equivalently, the other end of the chain will be a few inches below the rear triangle.
Then feed the inner tube over BOTH ends of the chain, and feed the entirety of the inner tube over the chain, so that the tube is bunched up and both ends of the chain are exposed. Get something thin and insert it through both sides of the chain so that it keeps the inner tube from sliding down. Then loop the chain around the bike frame and connect the chain.