"We are looking to use feathered timbers ontop of sterling board but there is one wall which will be totally out of sight and I wondered if we could use Bitumen corrugated sheets to cut down the cost."
You could but I'll bet you'd regret it in years to come.
"Also, are there any other options to feathered timber, apart from tongue and groove as that is obviously a lot more expensive."
Yes you could use UPVC cladding like Google "directplastics.com/media/catalog/product/cache/3/image/5e06319eda06f020e43594a9c230972d/F/o/Fortex-Pale-White.jpg" which comes in various colours as well as textures. For me that would be preferable to feather edge.
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