Delta,
As part of your cpl licence issue you must have completed a round solo trip flight of 300 nm or more with 2 full stops other than that of your home base.
I spoke to a number of people who told me that the CAA require a document signed by the CFI and stamped by the airport manager at each of the 2 other airports verifying the flight.
Ive since been informed by my CFI that a letter from the school outlining the details of the flight (dates, times, aircraft reg etc)and signed by the CFI will be sufficient.
Some people havent even been asked for this by the CAA whilst other have so its perhaps better to cover all bases..