Responsibility for gas leak damages

Other

My flat is a lease hold property. My tenants reported a gas smell and National Grid engineers identified a gas leak originating from the "internal piping" in the kitchen. The gas appliances and gas boiler are not at fault and I have an up to date gas safety certificate. Fortunately there were no serious consequences, but what would have happened if there had been a gas explosion or if the tenants had been injured or killed? Who is responsible for the safety of the internal piping? Is it the owner of the block of flats or myself? Should I take out insurance to protect me from damages caused by a such a gas leak? If so, can you recommend the best type of insurance?