CATIII,
Even if you can't find the exact dates, you can still claim. The online form gives you plenty of options to pick from.
You must complete ONE of the following options in order to receive a refund. Please choose the option which is most convenient.
Option 1: Provide passenger name, date of departure, origin and destination city or flight number for each flight for which you are making a claim
Option 2: Provide your British Airways or Virgin Atlantic Frequent Flyer Number
Option 3: Provide passport number (this Option available to British Airways Passengers only)
Option 4: Provide passenger name and ticket number