Did 750,000 people really attend the People’s Vote march?

If you think about it, it is obvious that The People’s Vote march last Saturday in London could not have been attended by 750,000 people, as its organisers allege. That is the equivalent of every man, woman and child in Edinburgh, Oxford and Cambridge (to take three Remain-voting cities). Sky News reported that the organisers claimed more than 500,000 (itself a preposterous figure), and by Monday this had swollen in most reports by 250,000.