It’s a long time since I’ve been back to the National Portrait Gallery and glided up the grandest escalator in western Europe to the restaurant overlooking the roofs of the National Gallery and down Whitehall.   I remember the moment when Jeremy Dixon and Edward Jones triumphantly described how they had persuaded a warder to let them out onto the roof, which I had never done, and discovered the greatest possible view of the institutions of British goverment to match the portraits below.   I was always a bit sceptical that people would find their way up to the top of the building for a cup of tea, but I was quite wrong.