The FTSE 100 Index is up 52.8 at 4417.6 at mid-morning.
Rolls-Royce is up nearly 10% or 12¾p to 146¾p after pleasing the City with half-year results at the top end of analysts' expectations, although they were down sharply on last year.
The aero engines group, which has been hit hard by the after-effects of the September 11 terror attacks, reassured the market over debt levels.
The Footsie had earlier climbed 72 points on the back of strong trading on Wall Street last night, but sluggish retail sales figures for July disappointed traders.
Forecasts of a slightly weaker start for the Dow Jones Industrial Average in the US this afternoon are also weighing on sentiment.
The host of blue-chips on the up include Vodafone, which has gained 3¼p at 109¼p; rival mmO2, ahead ¾p at 49p; and Cable & Wireless, which has moved 2¼p higher to 166¼p.
Barclays is up 9p to 483½p; Lloyds TSB is up 12½p at 608p; and Royal Bank of Scotland has climbed 21p to £16.51.
BP and Shell are both up 7½p at 529p and 448½p respectively, while AstraZeneca is recovering some of its recent falls - up 15p to £19.70.
Royal & Sun Alliance is up 7% or 9p to 127p on rumours that a European rival may bid for the life insurance business. Speculation also rose that chief executive Bob Mendelsohn had quit - a claim firmly denied by the group.
Fellow life insurer Friends Provident is up 2¼p to 154½p after agreeing a tie-up to sell products via financial services group Countrywide Assured, and to sell its estate agency division to Countrywide - whose share price is unchanged at 152½p after it reported a 56% jump in interim profits.