Hi and thanks for the comments. The bottom line of it all is that I do not want to leave him and I don't want to take a lover. I feel the value of what I DO have far outweighs what I don't. There is so much more to our lives - sex is only one part. The reason it's heavy on my mind is the lack of anyone to talk to about it - I don't know if I am odd in this respect, but I find it is really helpful to talk even if there is no "do x and all will be well" advice that I will be willing to take.
That doesn't mean to say that suggestions for improving things will be ignored - they won't, but they have to be ones which don't involve splitting up.