Should have thought to check another couple of sources before...
Mick Fleetwood's book mentions recording afer hours in the Decca studio (since Mike Vernon had a key), and states that some tracks had Bob Brunning on bass.
Bob Brunning's book goes into more detail. It also mentions the late-night recording sessions, and it sounds from the book as if they started around the time the band was rehearsing for their first gig at the Windsor Festival. He mentions his three released tracks as having been recorded at these sessions ("Long Grey Mare", "Rambling Pony", and "I Believe My Time Ain't Long"). The book makes it sound as if there was more recorded there than was ever released either on record or on the 4-song demo (he mentions recording "a great deal of material"), and it doesn't mention any recording going on prior to Jeremy joining the band. The book certainly makes it sound like the recordings used for the demo were completed prior to the band's debut gig, which means they all would have had Bob on bass.
Brunning's book goes on to say that Clifford Davis claims to own the original Decca tapes and the rights to release them, as he bought them from the Vernon brothers, but that Mike Vernon has no recollection of this deal. (BTW, I have the '98 edtion of Brunning's book, don't know whether that makes any difference).
So I don't know if that makes things any more clear or just more confusing, but take it for what it's worth
Here's a related question: exactly which tracks were recorded by Peter, Mick, and John during the session that produced the "Fleetwood Mac" instrumental? And when did that session take place? Again, there seems to be contradictory information on that. Both Mick and Bob's books make the very strange claim that "Curly" and "Rubber DucK" were recorded during the same session - seems highly unlikely unless Aynsley was there as well, and besides these two tracks are listed as being recorded in February and released in March '67. And were "Double Trouble" and "It Hurts Me Too" recorded at the same session as "Fleetwood Mac" and "First Train Home"? My guess is that they were probably recorded at a separate session around the same time in April '67, but that's only a guess. It just seems that since "Double Trouble" and "It Hurts Me Too" were "official" Bluesbreakers recordings while the other two were apparently not, it's unlikely that they came from the same session. And is it possible at all that "Fleetwood Mac" and/or "First Train Home" were part of that 4-song demo (don't know whether they would even have had access to these tapes at that time)?