Htc sent me these settings for an 02 simplicity sim card:-
On the device, tap the Menu icon (or Applications > Menu)
Select Settings > Wireless & networks (or Wireless Controls) > Mobile networks > Access Point Names.
Press the Menu key and tap New APN.
Select Name, enter O2 MMS Postpay and tap OK.
Select APN, enter wap.o2.co.uk and tap OK.
Enter the user name and password listed below. If no value is listed, leave the field blank.
User name: o2wap
Password: password
Select MMSC, enter
http://mmsc.mms.o2.co.uk:8002 and tap OK.
Select MMS proxy, enter 193.113.200.195 and tap OK.
Select MMS port, enter 8080 and tap OK.
If available, select Authentication type then select PAP
Select APN type, enter mms and tap OK.
Press the Menu key and select Save
The settings are saved and the service is ready for use.