CAP 437 states that when helideck temperature rises by 2ºC over the ambient temperature, the BHAB (British Helicopter Advisory Board) should be notified.
BHAB is the body in which the CAA discharges its regulatory responsibilities regarding offshore helidecks. Therefore, BHAB is the authority in this respect.
I used to inspect helidecks on behalf of BHAB, and in most cases when there was a rise in the helideck temperature it was due to hot exhausts. In any case, there was never a difference greater than 4ºC. In such circumstances the procedure in place was that radio room should pass on to the helicopter both OAT and helideck temperature, so that the crew were aware of the possible loss of performance once close to the deck.
CAP 437 also states that cold vent, exhaust and flare systems should be designed to terminate and discharge at a location on the installation as far from the helideck environs as possible. The potential release volumes, flammable concentration, and dispersal characteristics should be quantified, so that -if required- a 'no fly zone' around the hazardous area could be applied.
By the sound of it, the standards in the installation you mentioned above are nonexistent.
If you need more information about CAP 437, please let me know.
PS: There is a set of comprehensive studies (M E Davies - 1977 to 1979 - Local atmospheric environment of offshore installations), which was used at the time of amending CAP 437 in 1998. I don't have access to it now, but I guess it could be a very valuable source of information.