Someone else has posted similar problem. Probably has happened since Virgin network upgrade? I've an unlocked Q5 and Orange sim works fine in it. The problem definitely happens when SMS over 160 characters.
Hi sorry but I totally disagree with that as the problem was reported by other customers in the Forum with different handsets. The problem seemed to get resolved by the afternoon of 27th April. I hadn't changed any settings on my handset as a workaround.