Aero engine company Rolls-Royce reiterated its guidance for the full year but said the global trading environment remains 'very difficult', adding that the recovery is likely to be slow.Underlying pre-tax profits, which excludes the non-cash impact of the hedge book and other financial instruments, rose 9% to £445m in the six month ended 30 June.Rolls-Royce, which did better than most of its peers in terms of sales resulting from the Paris Air Show, said its order book increased by £2bn to a record £57.5bn (2008 year-end £55.5bn).Revenues rose by 27% to £5.14bn, a performance that was positively impacted by the weakness of the GBP against the USD and Euro. Underlying revenues improved by 17%, with a 27% growth in original equipment revenues (with all four businesses reporting a 20% or higher increase) and an 8% rise in service revenues. "The group expects that its global markets will continue to be affected by reducing demand and the impact of financing constraints," it said. "However, underlying revenues will continue to grow and underlying profits for the year are expected to be broadly similar to those achieved in 2008."