After spending far too long downloading old Yamaha manuals and comparing patch names with the ST-01 names I've come to the conclusion that my original guess that one of the sources was Yamaha DX11 is wrong. The DX11 came out in 1988 and the ST-01 sample set was released in June 1987 - D'oh!
I was right that it was a 4OP FM synth...
As it happens the DX27, DX100, DX21 and TX81Z all share similar sound sets and were all released before ST-01. On closer inspection the DX21's factory sounds bare most resemblence to some of the ST-01 sample names. I downloaded the DX21s factory sounds into my TX81Z via the most precarious of homemade tape interface leads via mp3! (yes it does work! - even with Spectrum games) Hey presto - there are MonoBass and HeiferBell without doubt
MonoBass sounds just fantastic on the real thing!
So far the Roland D50 is still in the lead with the most sounds being used for ST-01.
So far I've identified 30/128 sample sources and have leads on at least another 20.
The Korg sounds are still a mystery to me - they may be Poly-800 or DW8000.
Precisely which CASIO CZ was sampled is also still a mystery.
Answers on a post card