There have been a few cases of the hydraulic lines failing on the E89. Depending on where it is, if it's the one that goes to the top lock, it is a very labour intensive fix, as the line snakes it's way through a very difficult route.
Mine failed a couple of months ago, and Cayman Autos of Dorking (a hard top specialist), charged a very competitive £900 to change the hose. This was one that was made up by a company such as Pirtek, as opposed to a OEM replacement from BMW, that comes with the wiring/loom wrapped around it.
If you google it, there have been documented cases on the net, and the complexity of the fix.