Unmeasured staffs are confusing

[Joe Wilkinson]

    Hi Richard,
    I'm trying to typeset a Mandolin Orchestra piece with 8 staves where
    the first movement has
    3 opening bars unmeasured - timed at 10 s
    60 bars at 6/8
    7 Unmeasured staffs.
    
    I assumed, wrongly it seemed, that what I should do, in the top
    "Marks Staff" is to start off with an Time Signature >>
    Unmeasured Staffs(on/off), insert barlines where specified, after
    the last inserted barline use Time Signature >> Unmeasured
    Staffs(on/off) again, then set the time signature for the following
    bars.
    It did not work.
    So, to try an example I repeated the process in a new file on a
    single music staff.
    Unmeasured to start, then some arbitrary musac for 3 bars, inserted
    barlines. After the third barline Time SIg change to 6/8 and some
    notes (which were split correctly in the editor into 2 bars of 6/8)

    The typeset music was as below; the 6/8 in the fourth bar was not
    recognised in Typeset.

    Perhaps I have misunderstood; I looked in the Manual, but didn't
    find any help with Unmeasured staffs.
    Should I be using Cadenza time instead?
    best wishes
    Joe