DataGridView SelectAll

I am customising the top left header cell on a DataGridView to remove all filters applied to the data columns.

However the default behaviour of this header cell is to SelectAll if the DataGridView is mult-select.

How do I disable the select all behaviour of the top left cell while still allowing the grid to be mult-select.

Tried overrides on various methods of DataGridViewTopLeftHeaderCell but to no avail.

Hopefully I'm missing something obvious.

Thanks.