Highlight offset cell from target

[Drofidnas]

Hi

I have a table of 12 columns (a yearly report) in cells N1 to Y10 from which a graph is produced.
It's quite a complicated table but basically cell data is dependant on the content of the column 6 months previous (ie. P2 feeds V4).

I'd like to highlight the "source" area (P2) when click on a cell (V4).

I have this code which half works (not mine - I lifted it from the net).

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

With Target.Offset(-2, -6)
.Borders(xlDiagonalDown).LineStyle = xlNone
    .Borders(xlDiagonalUp).LineStyle = xlNone
    With .Borders(xlEdgeLeft)
        .LineStyle = xlContinuous
        .Weight = xlThick
        .ColorIndex = 3
    End With
    With .Borders(xlEdgeTop)
        .LineStyle = xlContinuous
        .Weight = xlThick
        .ColorIndex = 3
    End With
    With .Borders(xlEdgeBottom)
        .LineStyle = xlContinuous
        .Weight = xlThick
        .ColorIndex = 3
    End With
    With .Borders(xlEdgeRight)
        .LineStyle = xlContinuous
        .Weight = xlThick
        .ColorIndex = 3
    End With
    End With
     
End Sub

Two problems
1. There is no defined range and so the offset crashes when clicking less than 6 columns or 2 rows.
2. The border doesn't redraw with the original border and I end up with lots of red borders.

(Ideally I'd like to highlight the cell in a colour but the cells are already conditionally formatted so I'm guessing that would be too complicated for now)

Any help to polish this up, as ever gratefully appreciated.

Chris