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This is a good route. As mentioned some of the coasts roads after Cley aren't flat in the way the Fens are but we aren't talking the Peak District.
Peter C Said: Cley - Holt and then Aldeburgh (the Norfolk one) perhaps down to Aylsham. East Norfolk I am not so good on. Stay away from Yarmouth .
Aldeburgh (the Norfolk one) is very nice but I would skip Holt and Aylsham and just follow the coast. Great Yarmouth is a bit 'kiss me quick/lager lout' but the Sustrans route 1 goes right through it and is well maintained. You can also cycle through it in about an hour and the there are some good chip shops for refuelling.
Norfolk and Suffolk
You could start at Kings Lynn and then head for Wells via Sandringham , the Creakes and Walsimgham. There is a Campsite at Wells or Stiffkey. Unless you are up early avoid the coast road and from Cley ride the back roads up to Holt and then round the back of Cromer to Mundesley or Happisburgh. Beware, it's pronouned Haisborough.. Or you can get a train from Sheringham to Norwich.
Where are we now? Ah yes, in NE Norfolk. So drift down through the Broads and head south to Southwold. Great beachside campsite and you can avoid the A 12 by getting the ferry to Walberswick. Head S down the coast to Aldburgh and then you can get the train back to London from Darsham..