There's nothing particularly surprising about finding a west country landowner lying respectably in his own grave – except Sir James Tillie had been missing for 300 years, since his servants could no longer bear to carry out his last commands and bring food and wine daily to his rotting corpse.

Archaeologists have found human remains in a previously unknown vaulted chamber directly below the spot where Tillie sat for years after his death overlooking his estates.

The glory of Pentillie Castle, the home he built in 1698, is a spectacular view over the Tamar valley and river, and he was evidently determined to continue enjoying it after his death in 1713.

He left instructions that his body should be placed in an open windowed room in the mausoleum on the highest point of his land. He was to be left there fully dressed, positioned upright in his favourite chair, food and drink and a crate of books by his side, until the day of resurrection, which he expected to be imminent.

The mausoleum where the remains of Sir James Tillie were found. Photograph: PA

According to family legends, his servants faithfully obeyed the orders for two years, but when there was no sign of resurrection and the rotting body became intolerable, they buried him. There were no records of where, but he was believed to lie in one of the local churchyards.

Tillie's origins were comparatively modest. He worked as a land agent for Sir John Coryton, but became wealthy when his boss died – some said suspiciously early and suddenly – and Tillie married his boss's widow. The family continued to prosper and in the early 19th century the castle was extended. The renowned landscape gardener Humphry Repton remodelled the grounds, making the mausoleum taller and more elaborate, and blocking its windows.

The estate was inherited five years ago by Ted Spencer, a cousin of the previous owner, who has adopted the name Coryton. After he restored the main house as a wedding venue and hotel, grants from Natural England and the Country Houses Foundation allowed him to tackle the overgrown and crumbling Grade II-listed mausoleum.

The vault was discovered as part of a structural survey by Richard Glover, an expert in historic buildings, and the archaeologist Oliver Jessop. They found planks propped against the end wall planks – probably from a coffin, though Coryton thinks they could just be from Sir James's famous chair – and more on the ground covering human remains. There was no sign of a pipe, books or wine bottle.

Coryton toasted Tillie with homemade sloe gin from the estate before the vault was closed up again. "There are no plans to exhume the body or to undergo any further DNA tests," he said. "There is not really any doubt about who the remains belong to."

The exterior of the mausoleum can be viewed on garden open days and tours over the next few months.