I will post my usual response: get hold of a copy of the Good Pub Guide. In over a decade of living and travelling in Britain it has never let me down.
We ate at a lovely place in St Ives a few years back. Unfortuntely I don't remember the name, but it was directly across the street from the harbor, at the end near the lifeboat museum, upstairs. Very modern decor and menu full of fresh takes on local classics. I had a warm mackeral tart that lives in memory to this day....
If you are going to visit the Eden Project or Lanhydrock (both good takes), have lunch at the Crown, Lanlivery -- supposedly one of Cornwall's oldest inns, quite charming, and with excellent food. We counted 7 vegetables and 3 types of poatoes served with our lunch, which was seabass and very good.
Finally, if you are driving from the London area down to Cornwall, you can have a lovely lunch at The Kingsdon Inn, Kingsdon (near Somerton), Somerset.
Both pub recs we got out of the Good Pub Guide and they were really good!
Not sure how far Padstow is from St Ives, but head to Rick Stein's places - his original restaurant and also a cheaper bistro style place. Jamie Oliver has just opened a new '15' style place in Cornwall - Watergate Bay near Newquay, St Ervan Manor in Padstow or the Tressanton Hotel