Interestingly, Ian, the OP says he used gentle agitation for 10 secs after he first experienced problems but they then got worse???
Of course his" gentle" and yours and my "gentle" can be two different things.
To the OP I'd say that normal agitation and what I used for Ilford's Delta films and ID11 was about 4 inversions in 10 secs. How does this compare with your agitation pattern?
If Ian will be Holmes I'll be Watson so all we need is for the victim of the dastardly misfortune to continue to visit Baker Street and tell us more