The odd yellow leaf isn't much to worry about, but I do wonder when you last repotted it - sounds like its been in the same container for 3 years, so it may well be pot bound by now, so perhaps checking that would be a good idea. Also a general purpose feed in spring and about now, especially if you worry that you're watering too much. If you do repot, get some John Innes No. 3 to do it with, and a larger pot if necessary.

Thats great thanks very much. It is in a very large pot. I think I'll feed it then observe...perhaps repot later in the season, thanks again Bamboo