Meant also to say - if you can't get any reward availability, look into Yokoso fares (only available to non-Japanese residents travelling into the country on a OW carrier). These are priced at ¥10,000 each way, which (date and route dependent) can be a great deal. They are also upgradable to Class J for ¥1,000 (less than £7), though unfortunately only at the airport, so only if there is space available.