I want to make some batter to do Sweet and Sour chicken, but I've got no self raising flour.
Can I use plain flour with baking powder or bicarbonate of soda ?
If so, how much bicarb/ baking powder ?

Yes you can, use about 1 heaped teaspoon of bicarb in 200g of flour

We just use palin flour or cornflour, but I'd have though a spoon of baking powder would be better to give it a lift

I would go with baking powder as well. 2 teaspoons

I never buy self raising flour, so I use plain flour with baking powder when needed.

Bicarb would need some acid, I believe, but not sure.
I can always taste the bicarb when it's used on it's own.

If you only have plain flour you can try making the batter with fizzy water (sparkling mineral water or soda water will do) - it needs to be used quickly, though, before the bubbles disappear.

For extra crispy batter add some cornflour to the mix, as well as the wheat flour.