RAF merlins in St Albans?

I was at school(i wont specify) and it was break.My clroom is at the back of school and borders along Verulam Park,St Albans.Suddenly 2 RAF merlin helicopters painted gray flew low over the park.They circled for 20 mins and came as low as the tree line.Any idea what base they came from and why-eg if it is a common flight path.A chinook has ped low in november also.