i do install work for SG my understanding is that they only subby install work to approved contractors now, i.e big firms with offices and H&S, so I'm a subby to a subby
Install work is OK granted you dont get a suprise inspection, where they take about 2 hours out of your job quizzing you on technical stuff gas, electrical and safety related. Also it can take a while for you to get your "number" this is so they can score points against you for health and safety breaches, once you reach your maximum your off the contract or sent for more training.
They can be a pain in the arse that way but most big firms are when it comes to H&S