Smell of gas external to Condensing Combi boilers

If these boilers were lit at the time and putting flue gases through the vent I don't see how unlit gas could be the cause of the smell. Have you mistaken the smell of flue gases for the smell of gas? If you did smell gas in the garden it seems more likely that it would be coming from a leak in a gas pipe. The gas pipes would need to be underground in your garden and the gas would have an earthy smell. If you are convinced it was gas causing the smell you should phone the Transco emergency number immediately on 0800 111

It is normal for some unburned gas to be ejected from the flue during the ignition phase. It takes a finite time for the gas/air mixture to reach a combustable ratio and to find the ignition source, and whilst that is happening, the flue will be ejecting unburned gas. Combi boilers in particular have to ignite quickly at high power, and the methods used to do this are more likely to result in unburned gas being released during the fast ignition cycle, so if a hot tap is being turned on and off frequently (e.g. to rinse dishes), then you will have frequent ejections of small amounts of unburned gas.