Answers (2)
Some handsets will limit the amount of characters sent in an SMS message, however once you have gone over this limit it should then automatically convert the text to an MMS message, which could cost you around 40p for that message to be sent.
This can sometimes be changed in the settings, so have a fiddle around and see if that can be changed.
Otherwise - send two messages back to back? Or download Whatsapp and send it via that app, as there's no restrictions or charges for use.
Hope this helps. I work for a network provider and I deal with these kinds of calls from customers, just to let you know there's nothing the provider can do for this issue so I wouldn't waste time calling them.