We set the location of Triplet Hall in Corbridge, Northumberland, England. It's a small town about 17 miles to the west of Newcastle with a journey time by car of 30 minutes.
We envisioned the Hall to be on the site of Howden Dene Hall - now a residential care home. Triplet hall was to be far more grandiose however - something like this house we found for sale in Essex
Corbridge incidentally is the site of the Roman town of Corstopitum which was probably the most northern town in the Roman Empire. It's also believed to be the town on which Rudyard Kipling based 'Hunno on the Wall' in his book Puck of Pook's Hill. The Roman town has been excavated and is worth a visit if you are in the area.
If you should want to read Puck of Pook's Hill it can still be found as a printed book but is available as a free ebook from here.