Place the soy, sweet chilli, brown sugar and sherry into a non-metallic mixing bowl and mix really well. Add the ostrich and leave to marinate for at least 2 hours but up to 12 hours.
Place the potatoes in a large pan of salted water. Bring to the boil, cover and simmer for 15-20 minutes or until the potatoes are tender without breaking up. Drain and return to the pan over a low heat to dry out.
Heat a heavy-based frying pan with the light olive oil.
Drain the marinade from the ostrich and reserve. Pat the ostrich dry, then roll each side in the peppercorns. Add to the pan and cook for 2-3 minutes each side then allow to rest.
Mash the potatoes. Using a wooden spoon, beat in the butter until melted then mix in the wasabi. Lastly beat in enough of the milk until you have achieved smooth, creamy mash. Season to taste.
Add the remaining marinade to the hot pan and allow to reduce to a syrupy consistency.
To serve, pile the mash onto each plate, then top with an ostrich fillet. Finish with a drizzle of sauce and a sprinkling of chopped spring onions.