Gas appliance service leaves smelly finish

We selected Just Gas to service the gas appliances for the 20 units in our complex. After we drew up the time schedule for which unit to visit at any given time, they totally ignored this. Some owners had to arrange for parties to come open up their units because of this inconsiderate way of working. Apart from a cursory clean and quick dust off, I am not certain that the appliances were actually serviced. However, we took them at their word. Anyway, part of the deal was that upon completion, we would be given a certificate for insurance purposes. It's been 6 months now and we are still waiting for the certificate. It was promised that we would receive this upon the balance of payment, but once this was done, we have not heard a peep from them, despite an email reminder every other day. I would not recommend them.