Thanks for all replies. They told me to wait and see if there'll be a J seat on the flight tonight. But surely they know by now if there'll be a seat or not?! The latest I could fly would be Saturday. I wouldn't mind be put up in a hotel. More than likely all the J seats will be taken for the next few days and I should just fly home economy.
Another question....would I be entitled to enter the arrivals lounge at heathrow if I filed economy? Will they be able to see I originally paid J class?