I have a small leak from the nearside rocker cover near the rear, it drips down onto the lamba sensor. It is not oil, too thin, and not coolant, too dark, it is the colour of the red holomar gasket sealant. I realise it must be either oil or coolant but cant decide which, Anyone had this before ? Thanks.

Nope. If u are correct in what u say "small leak from the nearside rocker cover" then it must be oil, not coolant.
If u can decide and your looking at the leak how can we, the forum members.