Help Finding A Sunken U Boat
 

I have been reading a book called Destroyer Captain, all about a RN skippers exploits in WWII. What a life, one of the chapters deals with an attack on a u boat in the bay of Biscay, on 27/28 Aug 43, in company with HMS Jed and Rother, his ship (HMS Grenville) attacked with the other two dropping some 98 depth charges over a few hours. anyway oil was sighted in the water and bubbling up from beneth the waves, and a boat lowered from Jed found wood and bales of rubber floating in the diesel oil.
The author stated that some months later, the intelligence people told him that it was a boat returning from Japan with cargo needed by the Germans.
well I was intrigued by this and have attempted to find out which boat it was, in searching for it I went to uboat.net, which has a good data base of information, having looked under missing boats at around that time as I figured the Germans would have listed her as missing after she failed to report in. But I am unable to find a suitable candidate at all. I also looked at the Monsun boats that went to the far east, but no luck there either.
So hopefully I have wetted your'e appitites and ask if anyone out there can help me solve this mystery.
Thanks.

Japan also sent a few I-boats to the ports in France to deliver and pickup supplies, might have been one of them.