We have them as well.
After the last service we were left with a gas leak where they hadn't sealed the boiler up properly/noticed that the seal had been damaged when they were taking it off the boiler.
It took about 3 days for them to fix it, as they didn't have the seal in stock and it took 2 days for them to get it and fit it.
I will however say the callout time was pretty good, we had them out first thing in the morning, almost as good as the electricity board when the mains cable into the house fried itself (called them at about 8am, had a team out by 11am who were busy most of the day)