In pictures | McLaren’s subcontractor restores Ikea Oxford St windows

As a result, some 90 steel frames were removed to the ASWS workshops for shot blasting and repair. Those that were demounted also included the unusually shaped and very heavy 'lollipop’ lights. The remaining windows were serviced and overhauled, followed by decoration on site.

ASWS also restored 14 very large hardwood windows, with some needing new timber sections spliced into the frames.

All of the hinges and other hardware were replaced along with any broken panes. The unusually low positioning of the windows compared to internal floors meant all glazing under 800mm had to be switched for safety glass.

The three large shop fronts facing onto Oxford Circus at street level required a different treatment, as the huge plate glass windows were upgraded to safety laminated glazing and the elegant bronze frames were painstakingly stripped back to remove their green-tinted discolouration. 

The distinctive alloy was then re-patinated with an accurately matched chemical coating. The metalwork was then fully protected with an enduring wax coating.

ASWS also restored the three shallow height domes which form a feature over the fourth-floor mezzanine. The specialist contractor said this was one of the physically hardest jobs in the project because of hard to reach century-old encrusted dirt.

Kris Bennell, ASWS operations director, said: "The roof had suffered leaks, and everything required thorough cleaning before a lot of welding work was done over a two-week period, where joints had come apart or new steel sections had to be inserted to stabilise the lattice grid. 

"There were 148 panes of glass in each of the three domes, none of which complied with modern safety standards for overhead glazing, and also the glass had been given a very variable, sand-blasted finish. 

"It therefore all had to be replaced using safety glass, with each pane having to be individually templated by one of our carpenters - climbing in and out of what is a very tight space.  The reinstatement features authentic rebate and edge details, while each of the circular domes in their 5m x 5m frames received our five-coat treatment."  

The new flagship store for the Swedish furniture chain is one of the four quadrant buildings designed by Sir Henry Tanner, the principal surveyor of the then London Office of Works, which were built between 1913 and 1928.