TBH I don't think you have to go that far out of Norwich to get the rural ideal. I grew up in a village about 6 miles away, near the Broads and my childhood was almost like something from an Enid Blyton book (minus the tongue sandwiches ) My mother still lives there so I visit often and it hasn't changed much over the years. There are still a gaggle of kids playing cricket or tag or footy on the green, boys and girls from about 5 up to 12 all playing together.
Thorpe Green used to be really nice, if a little isolated if you don't drive, but it is probably about halfway between Norwich for the N&NU hospital and the James Padget hospital.
I am utterly biased, because I love Norfolk so find it impossible to be impartial and objective, but it can be an amazing place to raise kids.
As has been said though, it isn't terribly diverse. An oft told family tale is the time my Dad took me to Peterborough to get my first passport and I stood in the middle of a bus path asking why that man hadn't washed his face. I was about 5 and that was the first non-white person of any description I had seen (we didn't watch TV) Obviously it is more ethnically diverse now, but it still isn't anywhere near 'proper' cities in terms of ethnic mix. A minority in Norfolk is still very much that, especially in the sticks. That said, people are very tolerant and I have never had the impression that people don't immigrate there because they wouldn't be welcome. More that they can find far better places to go with more happening and more chance of work 
If you do move here, would your DP consider working in a care home for adults with learning difficulties? There are a glut of them in Norfolk (due to daft government white paper-ing, Norfolk ended up with a disproportionate amount of people needing care home places) so work in them is usually readily available and it tends to be a field that one doesn't need experience in (although obviously having kids is great experience at being a carer) as it is attitude that matters.
I wouldn't recommend Marham, purely because of the fact that the RAF base's future is up in the air and when Coltishal (RAF base about 8 miles out of Norwich, on the Broads) shut a few years ago it hit the area terribly and it went from being a highly desirable place to live, to somewhere you went if you couldn't afford the rent anywhere else. Lots of lost services etc which really impacted badly for the locals. I would hate to suggest you move to Marham only for the same to happen there.