Evening Andrew
You have got the correct Postcode, judging by the Royal Mail Postcode to Address Finder-
http://www.royalmail.com/postcode-finder
B R E National Solar Centre, Unit 1, St. Austell Business Park Carclaze, ST. AUSTELL, PL25 4FD
Crest Accountants LLP, Unit 11, St. Austell Business Park Carclaze, ST. AUSTELL, PL25 4FD
Ethical Power Ltd, St. Austell Business Park Carclaze, ST. AUSTELL, PL25 4FD
Nuco Mailing Ltd, St. Austell Business Park Carclaze, ST. AUSTELL, PL25 4FD
S G Accountancy, Unit 12, St. Austell Business Park Carclaze, ST. AUSTELL, PL25 4FD
St. Austell Printing Co, St. Austell Business Park Carclaze, ST. AUSTELL, PL25 4FD
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It appears that the mapping companies have it wrong, showing a group of houses immediately to the WEST of that small Industrial Estate.
If you Click on the RM Addresses above in that Finder, it comes up with-
"The map of the postcode you have searched for is currently not available."
Suggesting that the Industrial Estate is relatively recent - 5 years old or thereabouts.
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Most of the mis-labelled houses face on to the passing main road, Trenowah Road, with a dead-end lane going in behind from their eastern edge (with the Industrial Estate) to a few other houses.
At present, I have not found a name for that lane.
The west-most of those houses appears to be-
Google Earth Streetview
2 Trenowah Road
ST. AUSTELL
PL25 3EB
And its Map appears automatically on the Royal Mail Postcode site, but the Pin is just across that Trenowah Road, probably about the middle of the Royal Mail Delivery Walk defined by that Postcode.
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The above should give you the means of sorting out the confusion.
You should also confirm the Postcode at any of the Premises on that Estate already on Fibre; and try to obtain other information such as the PCP/Street Cabinet Number that his existing Phone Line comes through.
Also take photos of those cabinets and screen captures from the above Web Sites.