Scotland's economic recovery remains fragile and may yet suffer reverses despite surveys showing strengthening private-sector growth, business leaders warned.  Though the SNP has invested much hope in a rapid expansion of offshore wind power to drive the re-industrialisation of manufacturing, the renewables lobby has conceded that costs will have to fall sharply if the dream is to be realised.  The cautionary notes follow a business survey from the Bank of Scotland, which showed that manufacturing and service output rose in april more than in the UK as a whole. The index of activity in Scotland rose to 55.8, above the 50 level which indicates a standstill.