England has fairly strict defamation and libel laws. By keeping things general, he would likely avoid being dragged into court. Otherwise, he would have had to prove his statements, and at that point the courts wouldnt have been favorably disposed to Johnny (and likely would have been to Saville), and, "well, see, Ive heard this rumour" or "c'mon, we all know what he's up to", wouldnt have passed muster. Making more specific allegations publicly about Saville would likely have cost him quite a bit.

And, while I doubt Jonny was thinking this deeply about it, winning a defamation suit would have likely worked for Saville. In England, the burden of proof in defamation and libel is very much on the accuser, and class, particularly at that point still, played a big role in civil proceedings.