Thanks everyone for your help. Its at least pointing me in the right direction now.
With 35mm I tend to use are 35mm, 50mm and 85mm a lot of the time.
Same goes for my medium format stuff, my ETRS has the 75mm and the 150mm though I do toy with getting a 40mm for it. That covers pretty much everything I shoot.
We have some beautiful countryside in the Peak district that I'd really like to do justice of with a larger format neg. I'm guessing that the jump to 4x5 will be like the jump from 35mm to medium format.