Make the batter first by mixing flour and baking powder and mix in water and oil until you get a smooth batter. (I use a whisk to do this so its not lumpy). The batter should be thick enough to coat the potatoes, a bit like double cream maybe?
Leave the batter to rest for at least half an hour.
Slice potatoes into 1cm thick slices.
Half fill a wok with oil, heat till hot.
Heat oven to 180 oC
Dip potatoes into batter and deep fry until just starting to colour (a couple of minutes each side).
Take out and put in oven for about 25-30 minutes (or until potatoes are cooked).
Serve with plenty of salt and lot''s of vinegar.