I have had exactly the same problem. I phoned up on saturday to have a patronising woman tell me that a text is only 160 characters. I am fully aware of this, point is, i used to be able to send longer messages and the network would divide them into multiple messages as needed.
Basically after 3 phone calls, Steve in the Manchester call centre said it is a network issue they are now aware of but it is not affecting every customer.
I can only hope this is fixed soon and i take a tiny tiny bit of comfort knowing i am not the only one that is suffering from this problem. I don't want to have to phone up again as they still haven't fixed my voicemail issue...this has gone on for 6 months.