Visit Britain publish data relating to international visitors to the UK. They produce the data in two formats - individual spreadsheets for each region that are updated annually, and a single spreadsheet for all regions, containing less detail but updated quarterly.
Data shows London totals for nights, visits, and spend. Data broken down by age, purpose, duration, mode and country. This data is also available from Visit Britain website, including the latest quarterly data for other regions.
All data taken from the International Passenger Survey (IPS).
Data on accomodation occupancy levels is also available from Visit England.
An overview of all tourism data for London can be found in this GLAE report 'Tourism in London'
Further information can be found on the London and Partners website.