I said:

\starttypescript [TESTfonts] \definetypeface [TESTCombo] [rm] [serif] [palatino] [default] [encoding=texnansi] \definetypeface [TESTCombo] [ss] [sans] [helvetica] [default] [encoding=texnansi] \stoptypescript

I realise that by adding ---- \definetypeface [TESTCombo] [tt] [mono] [modern] [computer-modern] [encoding=\typescripttwo] \definetypeface [TESTCombo] [mm] [math] [modern] [computer-modern] [encoding=\typescripttwo] ---- to that typescript, I can stop the Deltas disappearing; however, it would be really helpful if someone can tell me why I only need to do that for font sizes below 12pt?! Thanks, Duncan