nokia 6630 on o2

hope someone can help.

it wont let me send a text.
no problem receiving text though.

when i try to send a text, almost immediatley a message comes up "message sending failed" the details go on to say "general: memory full. close some applications and try again"

but there are no other applications open.
i've tried re-booting on numerious occasions with no joy.

i'm using an o2 genie sim, but have tried a more recent o2 sim, aswell as a t-mobile sim. all with the same result.
all other functions work no problem.

anyone got any ideas??

cheers
john