Test the alternator output (places like KwikFit will usually do this free).
Check the connectors and make sure the relevant fuses are properly inserted, but I'm afraid it's most likely the dash is on its way out.
Copy and paste this URL
http://ecutesting.com/renault_megane_scenic_instrume.html
and this one
http://www.bba-reman.com/content.aspx?content=dashboard_instrumnet_cluster_range
and this one
http://3lib.ukonline.co.uk/scenic/trouble.html
(scroll to the bottom of the page.)
Do an internet search for renault dash repair to find others.
If you plan to remove the dash yourself it might be worth buying a Haynes workshop manual for your car. for instructions.
I suggest you also search and post your problem in the relevant section of
http://www.renaultforums.co.uk/
You will have to register, but it's free and very helpful.
bobo
April 2009