@SteveA -
'ff' was new to me too and like Mattrom I wanted to understand - Professor Google is little help.
The OED (1971) says:
"In MSS a capital F was often written as ff."
It continues " A misunderstanding of this practice has caused the writing of Ff or ff at the beginning of certain family names, e.g. Ffiennes, Ffoulkes."
Thusly, maybe ff was used cursively for F - presumably as it was easier with a quill pen.
Thereforth it perhaps became idiomatic and in usage amongst the writing classes, who would have used it (ff) to abbreviate common expressions like 'Fecerunt'.
Later interpreters, amongst them the Fiennes and the Foulkes families, then later still the Chambers dictionary people, looked back on the linguistic record and added two and two to make five?
Just a theorem!
For Ffoulkes sake!