* FTSE 100 up 0.1 pct
* Prudential rises after results
* Ashtead suffers double downgrade
* Slumps in Restaurant Group, G4S hit mid-caps
By Alistair Smout
LONDON, March 9 (Reuters) - Britain's top share index edged higher on Wednesday, boosted by a rise in insurer Prudential after it announced a special dividend with results that beat expectations.
Prudential rose nearly 3 percent, the top FTSE 100 riser, after its profit was lifted by strong performances at its British, U.S. and Asian life businesses.
"Overall Prudential remains in good shape and... has allayed investor concerns on management's ability to deliver on strategy," said Atif Latif, director of trading at Guardian Stockbrokers.
"The announcement of a special dividend and 5 percent increase of the ordinary dividend has been well received."
Britain's FTSE 100 index edged up 3.79 points, or 0.1 percent, to 6,129.23 points by 0855 GMT.
Among fallers, construction equipment rental company Ashtead was down 5.6 percent after it was downgraded to "underperform" from "outperform" by Credit Suisse.
"(We) conclude that, independent of the demand outlook, risks are building around rental rates," analysts at Credit Suisse said in a note, slashing its target price to 770p from 1300p. Berenberg and JP Morgan also cut their target price on the stock.
Mid-caps underperformed blue chips, with the FTSE 250 index down 0.4 percent, hit by disappointing updates from Restaurant Group and G4S.
Restaurant Group fell over 16 percent to its lowest in two years after providing a disappointing outlook for the forthcoming year.
G4S dropped 11 percent after it took an extra charge related to British government contracts.
"Security group G4S failed to woo investors with an increased dividend as attention remained fixed on losses on its government contract to house asylum seekers," Russ Mould, investment director at AJ Bell, said in a note.
Among risers, Cairn Energy rose 8.8 percent after announcing better-than-expected results from a well in Senegal.
Editing by Jeremy Gaunt.