Plaque

This recreated façade is the rear of the hotel on King's Cross Road. In 1969 the authentic Victorian houses were demolished to make way for the hotel and this was the best the local people could wrest from the developers.

Getty have a photo of the original Victorian building captioned "16 Percy Circus, on the corner of Great Percy Street in London, with a blue plaque commemorating the fact that Russian revolutionary Vladimir Lenin once lived there, UK, 13th December 1967." This photo shows the first plaque attached to the ground floor wall at the left hand side of the Percy Circus elevation.

Local History carries a very informative page on this area. 

British History Online explains that in 1905 Lenin and his wife lodged at 16 Percy Circus, 25 April -10 May, when Lenin was one of 38 delegates to the Third Congress of the Russian Social-Democratic Labour Party, held by the Bolsheviks, 'mostly in the backroom of some pub or restaurant', as he later recalled, to hammer out a strategy for the Russian revolution of that year. Other delegates were put up at Nos 9 and 23 Percy Circus.

At the prompting of the Finsbury Communist Party an LCC blue plaque was mounted in 1962. In August 1968 Nos 16–18 were demolished for redevelopment as an extension to a hotel, and the plaque was not allowed to be reinstalled on the new building, so it was given to the mayor of Moscow.

The developer of the site arranged for an unofficial replacement plaque to be erected. Unveiled in August 1972, in the face of anti-Soviet demonstrations, by the Soviet Ambassador, standing alongside the developer, it was immediately covered over again for fear of provoking further trouble. We do not have a date but it was presumably not covered for very long.

It was common to hold Communist meetings in pubs - see The Red Lion for some that Marx and Engels attended.

Other sources: British History Online. Wikipedia's List of former English Heritage blue plaques.