ok i got reply from o2 you need to manually edit settings to these if your a pay&go customer with a n900
as the n900 is not on there portfolio of devices they cant send them over sms

APN address: payandgo.o2.co.uk
Gateway (IP) address : 193.113.200.195
Username : payandgo
Password: password
Session type: Continuous or permanent
Authentication: Normal
Security: Off
Port number: 9201

you can set ip address to auto and leave port number blank, only settings i edited was APN, username and password and all works now on n900