Thank you to everyone who sympathised with my situation and recognised that I was trying to be respectful and not blame the person.
Just to answer a few questions - my husband and I had booked our seats early and paid extra so we could sit together and he could have an aisle seat, for his comfort, as he is tall. If I had asked to move there were no double seats available so we would be split up when we had paid to sit together and also I did not want to embarrass the person by asking.
However I was then annoyed when I realised the person was travelling with another woman who was sat two rows in front, so they had obviously not paid to sit together or had left it too late (the seat next to me was showing as unoccupied on the website only a couple of weeks before the flight) and she spent the flight in relative comfort.
Finally, as he was so large he did need a seatbelt extender and although the armrest came down he overspilled under and over it onto approximately 25% of my personal space, and although clothed, his body was touching mine from shoulder and arm to my hip and down to my knee, which as a woman made me very uncomfortable.
From the comments I can see that this is a complex problem, both for larger people and those who may have to sit next to them in a restricted space and I have read all the responses with interest. Clearly there are no easy solutions, but to say that if this bothers me, I should just not fly, is really unacceptable. Hopefully I will not encounter the same situation on my return flight but if I do I will ask to be moved even though I have paid for my seat.