hi Blackcraig 1. As far as I understand the underground scenes were shot down "Poldark" Mine at Wendron. Don't know where you live but that is between Redruth and Helston.
The original underground scenes in 1970's series were filmed underground at King Edward Mine (Camborne School of Mines training Mine section)
His farm house (Nampara) is on Bodmin Moor, but all the interior shots of his farm and peasant hovels filmed at St Fagans Welsh Folk Museum, just outside Cardiff. Truro is a combination of Charlsetown and Cosham, near Bristol. His uncles fancy house, now owned by Francis Poldark, is in Gloucestershire, but cannot remember where sorry.
The place where Demelza goes to her first ball would have been, in real life, the Assembly Rooms in Truro, which are still there by the cathederal. The Red Lion Inn where they are wheeling and dealing for buying copper, and Francis picks up "ladies of the night" and they gamble fortunes away, existedas a fancy 31 bed hotel until 1967 when a runaway lorry ploughed into it. It was demolished and a horrible concrete build replaced it and CO-OP built supermarket. The stairway from the hotel was saved and is in Godolphin House near Leedstown. 😉