Building GAS LEAK

Maintenance

The gas supply company visited the block where my flat is housed (suspect a call-out or periodic maintenance). They found a leak from my supply gas meter (communal cupboard G.Flr) and capped of my supply. They later informed me of the situation, a 0.4mb gas leak had been measured. I called an engineer who tested inside the flat and found no leak within the flat. However, on the visit he discovered that a number of other flats had also been capped off. His examinations were limited by lack of access to G.Flr garage area beneath the block, and the upper level where the penthouse area is. Never-the-less continuing his investgation on a second visit, he found a leak outside the garage area on an external riser (1 of 5 Riser pipes). With a number of flat's affected I consider this to be an infrastructure problem of the building.
CQ6 the maintenance fund management when informed will not accept responsibility and claim the responsibility of the leak is mine. I have asked for a copy of the maintenance contract but so far that has not been offered, nor have I been given any assistance in proving where the responsibility is. In the meantime my tenant has no hot water.
The maintenance contract was originally set-up by the builder and provided by a company in Manchester. Later the service switched to Berkley-Shaw in Liverpool who for a time dealt with all maintenance issues (Calls taken: Mon-Fri 09:30-17:00). At some point that service changed and they now operate only a 'front end' for the routing of faults to a management group named CQ6. CQ6 happens to be the original name of the site. No call numbers are available for CQ6 and initial contact must be made by referring to a website.
Can you advise in this situation as to how I may best progress the issues stated.