Heres an original Michael Ffolkes cartoon it has a handwritten to the back on headed paper dated 21st April 1978. The friendly letter to a Mrs Cecil explains that "The original is available for fifteen pounds. It is somewhat more impressive than the version that appeared in "Private Eye" measuring around 10" across". Im thinking this is the original cartoon referenced.
In image measures 10" by 7" approximately it is nicely mounted and framed, the letter is folded and taped to the reverse. Please study the photos
Framed dimensions 41 by 31cms approximately