Phone help needed!

[KernowPete]

Hi all, I know there are some knowledgeable folk on here re phones so am looking for some help before going through the pain of contacting O2. I have a Galaxy S6 Edge on O2 (contract not payg) but have just discovered I am unable to send MMS messages. I can receive them but not send. I get the message 'service not activated on network'. In all other respects the phone works fine. I can't remember if I've ever tried sending an MMS since upgrading from my S4 last year so no idea when this problem started. Grateful for any thoughts.....