That's lovely Alex
Can you put something over it to protect the nest, something with a large cutout at ground level, rather like a hedgehog home, maybe a piece of upsidedown guttering? I fear it's going to get trodden on or soaked. They will find the way in again.
Bees adore comfrey, don't they
Our patch positively hums with them, all bumbles as we have no honey bees here.