SELECT elements from column that are NOT in another column

I need to present a drop down menu to a user to select records that need processing, identified by the fact that the key parameter value is NOT contained in another table of a list of processed records. I am using the SELECT() function to compare as that seems most logical, however it only ever seems to return the full [unfiltered] list of records from the original column. This is my expression:

SORT(
SELECT(
Departure[TachoStart], ( Departure[TachoStart] <> Arrival[TachoStart] ), TRUE
), TRUE
)

I want to return records from Departure[TachoStart] whereby Departure[TachoStart] values are NOT matched with those of Arrival[TachoStart], as a match means the record [i.e. journey] is completed, using [TachoStart] as the key parameter that links both tables. I then want the records sorted in descending order.
Not sure how important it is, but Arrival[TachoStart] is defined as type text, as it is derived from a drop down menu, using data validation from Departure[TachoStart], that allows the user to choose the departure record to which the arrival info is to be appended, whereas Departure[TachoStart] is a decimal [as theyโ€™re all just numbers!!]. However, the expression assistant seems happy with this mismatch in data type, it just doesnโ€™t give me the expected output.

I tried NUMBER() but it didnโ€™t like that
( Departure[TachoStart] <> NUMBER( Arrival[TachoStart] )

whereas LIST() doesnโ€™t appear to do make any difference
( Departure[TachoStart] <> LIST( Arrival[TachoStart] )

I also tried FILTER() instead of SELECT(), but no difference, it still returns the full list
SORT(
FILTER(
โ€œDepartureโ€, Departure[HOBBS2Start] <> LIST( Arrival[HOBBS2Start])
), TRUE
)

help greatly appreciated. Thanks

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