Matthew is a 45 year old optician with a 4 month history of muscle weakness causing him difficulty getting up from chairs, combing his hair and going upstairs. The weakness is variable and is better on somedays and the lack of pain has meant he has put off attending the doctor up until now. He has normal sensation in his limbs and can type and write as normal at work. His speech and vision are unaffected.
Based on the history alone, what is the most likely diagnosis?