Try Ascombe S.G. near Teignmouth, Richard Chapple's at Broadmoor S.G. or Sid Vale near Sidbury.
The skeet range is still at Newnham Park near Plymouth but I think it's Corporate stuff or private hire only, but might be worth a try.
As you're in Cornwall ask Martin Elworthy at his gunshop. He's done a bit of Skeet in his time and would know more.