Even though this was in The Daily Mirror, it does have the ring of truth...

Via Drewster at HPC, from The Daily Mirror:

DAVID Cameron personally vetoed plans for a new property tax after donors threatened to ban Tory party fund-raising events from their mansions. Super-rich donors warned the Prime Minister that if he taxed their London townhouses and sprawling country estates they would refuse to host dinners to boost the Conservative Party's coffers.

A senior insider revealed: "The message went out - tax our mansions and you can forget us ever holding another black tie event for you at our homes ever again. Cameron could not have funded the party without these events so he had no choice. It was all highly embarrassing."

... Lavish Tory fund-raising dinners are regularly held in some of the most spectacular piles in Britain. But an insider said the message from Tory HQ fund-raising officials was that "almost all" big donors who offered their homes as venues for the party dinners were unhappy.

Lib Dem peer Lord Oakeshott accused Cameron and Osborne of "performing a hand-brake turn on donor orders". He told the Sunday Mirror: "The Tories are too scared to make their backers pay more than a £26-a-week council tax on their multi-million pound Mayfair mansions. That's not fair, it's feeble."

And no doubt dozens of Poor Widows In Mansions threatened to cancel their party membership subscriptions of £10-odd a year.