It may seem like expensive nonsense, but Rick Stein's chippy (eat in and takeaway) in Padstow has always been very good when I've visited.
The chippy in Polzeath is awful - avoid. Delabole one much better.
Of course you may be going to a completely different part of Cornwall...
I grew up in the "Haddock Triangle" of West Yorkshire, incidentally also within the "Rhubarb Triangle"...
(I am, of course, thus Dirty Leeds and a JCL)