Carolyn's Fruit Slice for 20

From the Forest School Camps - Sort of mince pie on a biscuit

You need a tray - 2 foot x 2 foot
Put 5 mugs of raisins and 5 mugs of sultanas (750g of each) plus an ounce of mixed spice, into a large pan.
Cover in tea - diluted a bit if you haven’t enough dregs.
Heat, but don’t boil or burn.
Add 1oz (25g) of corn flour and stir a lot until it thickens.
DON’T boil but keep hot
Meanwhile, crush 2kg of biscuits (digestives or similar) and add 1 pat of melted butter – press into the tin
Pour fruit over biscuit and leave to cool.