Thank you for the replies, altho I'm not sure I'm any the wiser.
The tree was in a smallish front garden, right beside next door's boundary. The stump was ground out but it was a big tree (over 20ft) so goodness knows how far any remaining roots go.
I have perennials and grasses in the bed now.
Should I clear the bed?
What is susceptible?
If it's harmless why do you hear so many horror stories?