Wowbagger - we were a mile or more away from what used to be the official 'clothing optional' beach. I should have mentioned at the time...
In answer to the original question, Peterborough - NCN 63 to Wisbech (pronounced 'Wiz-beach') then NCN 1 to Wells
Next The Sea, gives you an almost-entirely-on-quiet-road route, although locals would probably by-pass a few of the rougher roads/shared paths.
It depends where in the 'Peterborough area' you're starting from. I'm normally crossing from North of Peterborough, so pick up the old Thorney Rd from Peakirk through Thorney, then use minor roads each side of the A47, with a couple of 100m A47 bits to get to Guyhirn. From there if you head towards March, you can pick up NCN 63 to Wisbech again (I haven't done March to Kings Lynn section, but ridden the rest).
North Norfolk is hillier than you would expect, but a delight. Going through the grounds of Holkham Hall is a highlight of the ride.