Thanks all.
To clarify, I have tried to use the 'slot' rather than the 'round' hole, but my flat cable is too big.
Given that the cable has a 'thin' side and a 'thick' side, the 'thin' side fits in the slot ok, but the 'thick' side is too thick to get the rear panel back in place.
Ie The cable sticks up too high by approx 1mm.
So apparently not all flat ethernet cables are of the same dimensions.
Hence my query as to what make others have purchased.
Cheers,
Vern.