Yes - they were hired from E.H. Brakell. From the 1981 edition of Capital Transport's "London Transport Buses"...You're right about the 12 that LT purchased, but a little later LT operated 4 on their Round London Sightseeing Tour, though these were hired in, I believe, and may have included a couple of the previous rejects.
"Sixteen front-entrance Leyland-engined Routemasters were bought from Northern General and via dealers between December 1977 and July 1978, seven of these passing to London Transport in December 1979. Of the remainder, five were sold and four were numbered RMF2761-2764. During 1980 these were renumbered 2791-2794 so as not to clash with London Transport's RMFs. A fifth vehicle was acquired from Booth's of Rotherham in December 1980 and is expected to be numbered RMF2795. RMFs 2791 and 2792 have been used regularly on the Round London Sightseeing Tour during 1979/80 and RMF2793 has been converted to open top. This and RMFs 2794/5 have yet to be used."