* One-sentence rebuff to speculation driven by blogger
* Quindell stock up 28 pct; National Accident down 5.4 pct
* Top-10 fund investor backs firm, confident on cashflow (Adds bullet points, investor comment, updates shares)
By Noor Zainab Hussain and Simon Jessop
Nov 20 (Reuters) - British technology and outsourcing company Quindell denied on Thursday it was actively seeking to sell a 25 percent stake in Nationwide Accident Repair Services sending its shares soaring more than 30 percent.
Quindell, which provides technology used by car insurers to assess claims among other services, issued a one-sentence statement in reply to comment from blogger Tom Winnifrith that the company was "desperate" to sell, and was seeking "any offers at all". (bit.ly/1xFj614)
NARS declined to comment on Quindell's statement, while Quindell did not respond to requests for comment.
Quindell's heavily shorted stock, among the most volatile on the London Stock Exchange in recent months, rose as much as 32 percent to 56.75 pence in morning trading before retreating slightly to trade up 28 percent at 1402 GMT.
NARS' shares were down 5.4 percent at 71 pence.
Quindell has had a tumultuous few months, facing allegations about its business model and questions about the motivations behind some acquisitions.
The company's founder and chairman, Robert Terry, stepped down on Tuesday. Terry's resignation came days after Quindell disclosed a complex share transaction involving its chairman and two other directors, who will also leave the board
"We've had some foolish mistakes from the directors," a top-10 fund investor said, "but they do not have any direct affect on Quindell's cash flows."
He added that he had maintained his holding during the last two weeks.
"I expect the company to report the operating cash flows for the final quarter of the year which it has guided to," he said, referring to the 35-40 million pounds($55-63 million) flagged by the company for the second half of the year.
One of the biggest institutional investors in the company, Fidelity Management & Research Co, said it had halved its stake on Nov. 11, a day after the company disclosed details of the share dealings. (bit.ly/1ySgn18)
On Monday, Quindell said Canaccord Genuity had resigned as its joint broker. The reason was not disclosed.
Quindell's problems started in April when Gotham City Research called into question the company's revenue model and profit quality. The stock has never recovered, even though Quindell won a libel suit against the U.S.-based short-seller.
Quindell has lost more than 2 billion pounds ($3 billion) of market value since April, and as of Wednesday it was valued at 188 million pounds. NARS was valued at 32 million pounds.
1 US dollar = 0.6387 British pound 1 US dollar = 0.6360 British pound Editing by Ted Kerr