His mother is Gillian Thrale, father George (surname unknown). His maternal Grandfather is Richard (Dick) Thrale b.1885, who was also a horse trainer.
Other than the two links here, I don't know anything else. I've never met them and I have no idea where they fit into the Thrale.com family tree.
If you know anything further do let me know. If Richard Dunwoody or one of his family reads this, do get in touch, I promise Thrale.com will respect your privacy.