We have now searched the muster rolls of almost all units present at Quebec in 1759 and Isaac Eyles Warren does not appear in them.
On the gravestone at Warham, Norfolk, he was described as 'a volunteer'. This could mean he was an irregular soldier, not officially in any regiment, I suppose.
However, I think that it's more likely he was in a unit for which muster rolls do not exist for the period, or for which no search has been made. There are no rolls for the 78th Foot (Fraser's Highlanders) or the 60th (light infantry).
There ARE rolls for the Royal Artillery but time did not permit me to search properly. They are not arranged quite as the infantry rolls and it would take time and patience to go through them thoroughly. I refer to WO10/70.
If anyone needs to look at WO10/70 for 1759 - 1760 please keep an eye open for Isaac Eyles Warren! - if not, I'll be back next October.