It's pretty rare for a stutter to develop at this age but not unheard of. Many people say their stutter is affected by their sleep. First thing I would do is seek out a speech therapist. I dont know what how they certify them in the UK but in the US you would look for a board certified fluency specialist. You should find one with an expertise in stammering because most speech therapists don't really know about stammering. In the meantime some basic but important points: 1. Don't try to hide your stammer. It's going to reinforce a fear and shame about stammering. 2. Don't try to push or force out sounds if you block. Tell yourself that you will give yourself time to get sounds out and there is no rush. you dont need to put extra pressure on yourself. Take it from someone who's been stuttering a long time. The truth is. no one really cares if you stutter. Yes, it is annoying and sometimes very frustrating. But be kind and patient with yourself and move through it as you can.