SIR Robin McLaren looked crestfallen when no one asked him about his new grandchild. The normally dour ambassador had positively glowed with pride when he announced as a ''a good omen'' for the Sino-British talks the arrival of the latest addition to the McLaren household. But no one seemed interested. Sir Robin's adversary, Vice-Foreign Minister Mr Jiang Enzhu, did at least offer his congratulations but the press corps at the opening of the third round of talks at the Diaoyutai State Guesthouse had other things on its mind. ''Will you be discussing concrete issues?'' he was asked. Sir Robin, evidently put out that his grandchild had been ignored, glared at the questioner and replied in his most condescending manner: ''We always discuss concrete issues''. Back at his residence after the first morning of negotiations, the proud grandfather admitted he had been ''very disappointed'' that the issue of his progeny had not been commented on at the opening of the talks. After skilfully avoiding questions about the negotiations at his lunchtime press briefing, Sir Robin was only too eager to furnish those willing to listen with details of his ''fine, strapping'' grandson. His eldest daughter, Emma, 26, had given birth to the 3.4-kilogram boy at London's Ealing Hospital on Thursday, Sir Robin said. Yesterday's business began at 9.30 am when Sir Robin's gun-metal grey Daimler swept up the drive and came to a halt outside Diaoyutai's Villa No 10, location of the first round of talks in April. In a perfectly choreographed move, he and Mr Jiang met and shook hands in the centre of the plastic wood-lined entrance hall, directly in front of the negotiating room. Mr Jiang was all smiles as he opened proceedings with a positive commentary on the weather. and a hint that the differences between the two sides might not be that great.