Whisk eggs and castor sugar until thick and creamy. Fold in flours.
In a bowl drain mandarin slices. Save half the juice. Mash until a pulp. Add to eggs sugar and flour mixture. Add saved juice.
Grease pudding steamer. Place mixture in pudding steamer. Place lid on top. Place steamer in slow cooker. Fill up slow cooker with boiling water until half way up sides of steamer.
Cook on high for 3 hours. Now and then check water level and top up if need be :)