Streetly Electronics Mellotron Tape Library
Yes, the old wheezing and groaning 'cello is still with us. If there is a sadder sounding instrument better suited for the Mellotron then it's yet to be found. (Note that by sad I mean 'melancholy', not 'ukulele')
The variant effect on the lower end of this sound is entirely due to the 'cellist who recorded the original sound for Streetly. Reginald Kirby - a noted string sessionist in the 1960s - did not want to detune his 'cello to the lower register on the Mellotron keyboard for fear that he would damage it. For some reason this unlikely explanation was accepted and instead, a double bassist was used to play the lowest G to B range, hence the 'booming' effect and extreme timbre alteration.
Oasis famously used this sound on 'Wonderwall'.