Is this Britain's most expensive estate?

What's believed to be the UK's most expensive house on the open market is up for sale - though it's hard to be sure, as the agents refuse to reveal the guide price to anyone that can't provide references and pass financial checks.

However, the sellers of Hackwood Park House, near Basingstoke, Hampshire, are believed to be looking for at least £65 million for the estate.

Built in 1680, the main house has 24 bedrooms and 30 bathrooms, and sits in 260 acres of grounds. It's designed, say agents Savills, to be run invisibly by the staff, 'having numerous staircases and ancillary rooms to allow staff to discreetly pass between family and entertaining areas'.

The main house extends to over 51,000 square feet, and there are two gatehouses and four estate cottages as well as stables and a coach house.

It has been extensively renovated - since these photos were taken the last time the property was sold - and now includes a gym, indoor pool and spa with hot tub, as well as a wine cellar.

"Hackwood Park has been restored to its nineteenth century grandeur complete with original detailing and craftsmanship," says Crispin Holborow of Savills. "Whilst infinitely beautiful and grand, it readily fulfils its function as a home for entertaining."

Outside, the grounds include a botanical garden laid out in the late seventeenth and early eighteenth centuries, as well as a deer park, a lake and an ornamental eighteenth century woodland garden.

The estate began life as a royal deer park, but was most recently lived in by Lady Camrose, the mother of the Aga Khan, who passed away in 1997. It was sold a few years later for £14.5 million.

The identity of the current owner isn't known, but is believed to be a foreign billionaire who bought the house through a British Virgin Island-registered company.

Potential buyers are being warned that, 'due to the sensitive nature of this property' they may have to sign a confidentiality agreement before they can even get more information on the property. They'll also have to prove their financial status and provide references before viewing.

Last summer, it was widely reported that one Knightsbridge house had sold for a jaw-dropping £280 million, making Hackwood Park look positively cheap. However, that sale is shrouded in secrecy, with the house never having been placed on the open market.