Thanks guys.
I have received the following from Tascam support - a very prompt and useful reply:
Thank you for contacting Tascam U.K.
The 2GB capacity displayed is the maximum file size that can be recorded at one time - this is set by the WAV format.
Once this limit is reached, the unit automatically and seamlessly creates a new folder and recording
To view the card capacity and what space has been used, you need to go into the information screen on the unit.
Kind regards
Jo Fialho
Tascam U.K Support