At the time of Gilbert & Sullivan and as late as when the Gherkin was built, there was no 70. When I first tracked the Gherkin down and realized the street, I think the highest address on the block was 60. Then on a later trip to London I noticed a Starbucks had the 66-70 address, and now I understand a "Can of Ham" skyscraper will be 60-70 St. Mary Axe.