Just had this pop up today on it's own , but with message to turn engine off .
first long journey since Lock down .. and it has been so Hot 32 degs

i have checked Hydraulic Oil in reservoir, it looks a little low just below Half way , but thin and black.
is it an easy task to flush and replace this oil ? before i start to spend big money on replacing other bits ?

When you say "just below half" do you mean the halfway line between the two marks at the top of the reservoir? Not halfway up the reservoir. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

yeah , Half way up the reservoir , i am guessing that is too low

Yes. There are three lines near the top of the reservoir that give the min and max fill levels with a middle line for good measure.

A bottle of Cold Climate PAS fluid, top it up to middle line, run it and then see what the level is. Top up as necessary. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed