so the starter clicks? sounds like your battery is knakard or the starter is knakard try rocking the car in gear or bump start it.
economy mode will have nothing todo with it as you need to start the engine to get it out of economy mode.
and dont short the cables together this has been known to set air bags off and all kinds of problems as capacitors still keep things turned on

Is the battery defo charged it should be 12.6v even if its at 12.2v its only 25% charged.

It says something about fuel additive?

Will guarantee its gone into economy mode, deal with it day in, day out.

Does it say Economy mode active on the dash display?

starter motor?

It doesn't even turn over, will unlock, key in dash all lights on. Next click... Nothing...

if that battery is charged it wont be a problem with the alternator, is the car trying to turn over? if not sounds like new battery

Water in alternator ? Never heard that ?

How do you short it?

Is it saying economy mode on the dash? If so short the starterbwith the ignition on and hey presto. Will start on the key after that.

My C2 has been sitting for a little while and wouldn't start, charged the battery, still nothing.

Apparently it can be due to water getting into the alterantor? Would a jump start/bump start fix this???

...and yes I know its french.