“Sunset” will come a little late this summer.Sir Andrew Lloyd Webber, composer and producer of the new musical “Sunset Boulevard”– already the hottest ticket of the West End year — announced Thursday that the show’s opening night will be postponed two weeks to July 12, with a corresponding delay in previews, which were due to start next week. The reason, it seems, is hydraulic. As part of a $ 2 million refit of the Adelphi Theatre for the show, a new system of stage lifts was installed with, it now appears, just one little snag — the system responds to calls from all forms of radio telephone. Anyone in the vicinity, including cab drivers, who uses a remote mobile phone is liable to trigger four tons of scenery, which can then move at random and in any direction, often that of the cast. In the interests of company safety, the system is now undergoing drastic technical revision. Ready for my engineers, Mr. DeMille.