Motor boating can be caused by several things including bad/leaky decoupling capacitors.
See Post # 7 of this link :-
https://www.google.com/url?client=in...PymiV0wgQsMSiv
I have never done this fault finding technique of adding a capacitor in parallel, if attempted be very careful as some of the decoupling capacitors will have appreciable DC voltage.
I cannot lay my hands on my Ferrograph 2A schematic but wonder if your paper C10 could be a decoupler/bypass, if so then definitely worth checking/changing.
David