Even though you are in England, our government watchdogs give you a good 2 hours to fiddle with your food before there is a risk of such explosive bacterial growth that the food becomes potentially dangerous.

In general food safety terms, it generally takes 4 hours before bacterial growth reaches the danger point. You don't want your food to linger between 40°F and 140°F (5°C and 60°C) longer than necessary, but even if your preheated oven sank below 60°C in the 20 minutes that it was off, the chicken was in the "danger zone" for such a short time, you should have no worry.