By Mark Jefferies & Laura Harding ABBA and their fans celebrated the 50th anniversary of their Eurovision success in style at the weekend – with Bjorn Ulvaeus making a guest appearance in London’s West End and saying he feels “humbled” by their continued popularity. Saturday was 50 years to the day since ABBA won the Eurovision Song Contest in 1974 with their mega-hit ‘Waterloo’. Bjorn was at Mamma Mia’s 25th West End anniversary, whilst over in East London there was a live show for the digital version of the band at Abba Voyage too, with big celebrations. Taking to the stage after a celebrato…