Hi NMC20 

Another Power Query solution

let
Source = Your_Source,
Unpivot = Table.UnpivotOtherColumns(Source, {"Item"}, "Attribute", "Value"),
Group = Table.Group(Unpivot, {"Item"}, {{"Data", each _, type table }, {"Sum", each List.Sum([Value]), type number }}),
Expand = Table.ExpandTableColumn(Group, "Data", {"Attribute", "Value"}, {"Attribute", "Value"}),
Percentage = Table.CombineColumns(Expand, {"Value", "Sum"}, each _{0}/_{1}, "Percentage"),
#"Type %" = Table.TransformColumnTypes(Percentage,{{"Percentage", Percentage.Type}}),
Pivot = Table.Pivot(#"Type %", List.Distinct(#"Type %"[Attribute]), "Attribute", "Percentage", List.Sum)
in
Pivot

Stéphane 

What is the original format of your data? From what I'm gleaning, it seems like you have more of a modeling problem than anything else.

For example, based on the table you provided, I would put together a model along the lines of:

Tables

Items

Availability

Time Slots

Uptake

Dates

Dates = 
GENERATE(
    CALENDARAUTO(),
    ROW(
        "Year", YEAR( [Date] ),
        "MonthNo", MONTH( [Date] ),
        "Month", FORMAT( [Date], "mmm" )
    )
)

Model

You can then use a visual matrix to get what you are after using the following measure:

Uptake / Available % = 
    VAR _uptake = SUM( Uptake[Uptake] )
    VAR _available = SUM( Availability[Available] )
    VAR _openSlots = CALCULATE( COUNTROWS( 'Time Slots' ), 'Time Slots'[Open] )
RETURN
    DIVIDE( _uptake, _available * _openSlots )