Perhaps "modified Lancasters" refers to the Lincoln? According to Wiki/National Archives these were involved in developmental tests dropping the Blue Danube casing:
"Lincolns were frequently employed as
testbeds in new
jet engine development.
RF403,
RE339/G and
SX972 flew with a pair of
Armstrong Siddeley Python turboprops outboard in place of the Merlins, and was used for the ballistic casing drop-test programme for the
Blue Danube atomic weapon.
[29] "
The National Archives, London, file ES 1/44