Apparently when the artist heard about the demonstration against Blair + Bush's special relationship over the 2003 invasion of Iraq, he covered the two figures with a black sheet and did just that - as a protest.
The tale is told on Bench Poetry:
"When millions marched against the invasion of Iraq, I placed a black satin sheet over the statue, and sat beside it. To passers by, I said: ‘This is a protest against the 'special relationship' that Bush and Blair have trashed.’ The police asked me to remove the sheet (‘this is a public monument!’) and I complied – but only until they were out of sight.’"
from http://benchpoetry.blogspot.com/2008/11/allies.html