HC dismisses Revenue appeal, appropriate tax rate for works contract service would be rate prevalent at time of service rendition, not receipt of consideration; Rejects Revenue’s contention that by virtue of CBEC Instruction dated April 28, 2008, assessee liable to pay composite tax at revised rate of 4% instead of 2% since payment received post March 1, 2008; Foundation of Revenue’s argument incorrect since said Instruction held invalid by HC in Vistar Construction case; Moreover, disallows Revenue to raise plea that Vistar Construction case distinguishable since it failed to consider Rule 3 of Works Contract (Composition Scheme for Payment of Service Tax) Rules, 2007; Absent any allegation in show cause notice or argument before lower appellate authority, Revenue cannot take such plea for first time at HC stage : Delhi HC