For those who were talking about MHA section, sections only cover mental health assessment and treatment, not any other treatment for physical health. As others have said, the crux here is a capacity assessment (Helenia is spot on with Re C). if you can't get someone to talk the patient round and reason with them and you think they might have capacity to refuse, then you'd be getting seniors in and getting the Trust lawyers / your defence union's advice.
Not so long ago I was trying to put a chest drain into a patient with schizophrenia who had a self-inflicted pneumothorax from setting off a powder fire extinguisher in his face. He was acutely psychotic, had already had enough IM meds to flatten a small elephant and was still kicking and thrashing round the bed. Time was off the essence, so I had a small army of nurses who were holding him still and at the same time reasoning firmly with him and talking him round, and that did the trick and the drain went in. For about 4 hours. Till the meds wore off and he pulled it out and we had to do it all over again... *sigh*