The mosaics were first professionally cleaned in 1930, some 45 years after their installation. By the 1990s, however, they were showing signs of neglect. Expert surveys identified that large areas had become discoloured and that in certain places the substrate was decaying.
Given their cultural signifiance to the nation, the then PCC set up a sub-committee (under the chair of Veronica Donkin) in 1998 to investigate possible solutions and costs. The final decision was to take measures to 'conserve', rather than 'restore', the mosaics - at a cost of £50,000.
The work took expert conservationists several months to complete, the scaffolding finally coming down in early 2006.