Red-breasted Flycatcher was first recorded in Hong Kong fairly recently but has now been recorded on a few occasions. Bear in mind though that the vast majority of birds will prove to be Red-throated Floycatcher, which is a fairly common winter visitor.
Key features to look for include:
Uppertail coverts (jet black in Red-throated, browner in Red-breasted)
Bill (all black in Red-throated, pale base in Red-breasted)
Call (a very rapid rattle in Red-throated, slightly slower in Red-breasted)
There are a few other features, but I can't remember these - hopefully someone can post later to detail them.
Based on the uppertail and bill, your bird is Red-throated. The groups of planted trees around Long Valley are a good site for this species.
I hope this helps. Hopefully someone else can provide a few more details later.
