How to improve reception in a wireless CCTV system?
These problems are notoriously difficult to solve, especially if you're not on site. Going with the idea that the steel cladding is the major problem one solution may be use a passive aerial system to retransmit the signal.

Basically it would consist of two (2.4Ghz) aerials connected back to back with a short length 50 ohm wire. One aerial to the inside of the building, the other outside. The internal aerial would pick up the transmission from the cameras and the external aerial retransmit it. The system requires no external power source.

(see http://www.radio-active.net.au/web3/80211/Antennas/Passive )