After searching various places for how to change my 2008 Sirion headlamp bulb (H4 dual filament) I couldn't find any help.

Plenty of other 'hatsu tutorials showing removing arch liner and bumpers, similar for yaris etc. requiring to release the whole lamp unit to swap.

No need for the Sirion.

Three pronged connector plug off the back of the bulb under the bonnet (this took quite some wiggling and force for both mine)
Remove dust/splash guard.
Unclip retaining spring.
Remove bulb.
Replace bulb making sure it aligns correctly.
Replace retaining spring clip.
Replace dust guard.
take a moment to check the alignment of the prongs on the bulb to asssit in lining up the connector plug.

Done, literally a 5 min job for both.

Hope that helps someone in the future.